Add PROCESS.md documenting the new SDLC

PROCESS.md

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+# JSON Schema Specification Development Process
+This document formally defines the process used to evolve the JSON Schema
+specification as of the first stable release in 2023. It applies to the core
+specification and the specifications for any dialects and vocabularies
+maintained by the JSON Schema Org. It does not necessarily apply to everything
+the JSON Schema Org maintains. For example, media type registration follows the
+IETF process. Certain components used within JSON Schema, such as Relative JSON
+Pointer, may also follow a separate process.
+
+## Canonical URLs
+There MUST be a stable canonical URL for referencing any specification that
+follows this process. If the spec is made available from any other URLs, they
+SHOULD redirect to the canonical URL. If the canonical URL is changed in the
+future, all previous canonical URLs MUST remain accessible as redirects to the
+current URL.
+
+## Mutable Spec
+There is only one version of the specification and it can be updated at any time
+as long as those changes don't violate the compatibility rules identified by the
+stability model. Bug fixes, clarifications, and other non-functional spec
+updates can be merged at any time even for stable features. A change-log blog
+post SHOULD be prepared quarterly to give visibility to any changes that were
+merged in the last quarter.
+
+## The Stability Model
+Every feature in the spec has a release status. It's either evolving, stable, or
+deprecated. Flags are used to show the status a feature is in. If a feature
+doesn't have a flag, it's considered stable. If it has a year flag (such as
+`2024`) that's the year the feature reached stability. The other flags indicate
+a feature that is not yet stable or is deprecated.
+
+### Champions
+Any feature that is evolving SHOULD have a champion. The champion is responsible
+for shepherding the feature through the process to into stable status. The
+champion doesn't need to do all the work, they're just responsible for making
+sure the work gets done. The champion role MAY be shared by more than one
+person. An evolving feature without a champion SHOULD be removed from the spec.
+
+### Proposals
+New features start out as proposals. Proposals are informal GitHub Issues or
+Discussions and can be submitted by anyone.
+
+All proposals MUST be backward compatible with the current stable specification.
+Features that could be problematic for forward compatibility are highly
+discouraged, but may be considered if the feature is optional and disabled by
+default.
+
+**Promotion Checklist:**
+[] Consensus among JSON Schema Org core contributors that the proposal is worth
+  pursuing
+[] The proposal has a champion
+[] A PR for an entry for in the `STAGE-0` registry has been approved
+
+### STAGE-0
+`STAGE-0` features are tracked in the `STAGE-0` Registry on the website. This
+registry lists all `STAGE-0` features. Each entry SHOULD include the following
+checklist as well as an informal description with at least one example and any
+relevant links.
+
+This stage requires at least two implementations to implement the feature. The
+purpose of these implementations is to identify any potential implementation
+issues that might lead to low adoption such as difficulty of implementation or
+performance implications. Therefore, these implementations don't need to be
+publicly available or have real world use at this point.
+
+**Promotion Checklist:**
+[] There is general consensus among the core contributors for adding the feature
+  to the spec
+[] Tests are available in the test suite
+[] Two implementations have implemented the feature and pass the tests suite
+[] A blog post has been published introducing the feature
+[] A PR for the spec has been approved that completely specifies the feature and
+  removes the feature from the `STAGE-0` registry.
+
+### STAGE-1
+`STAGE-1` features are included to the specification, but flagged as `STAGE-1`
+and are not subject to the compatibility requirements of "stable" features. They
+can be added at any time as long as it meets all criteria for `STAGE-1` status.
+
+At this stage, the feature should be seeing real world use. These features MAY
+change or be removed all together based on what we learn from real world usage,
+but anything more than small changes should be extremely rare. Any necessary
+feature experimentation and evolution should have be done in `STAGE-0`.
+
+Implementers are encouraged to implement `STAGE-1` features, but are not
+expected to maintain support for previous versions if they change. Users who
+choose to use these features should be prepared for the possibility of changes
+and be willing to update their schemas if a change does occur.
+
+A `STAGE-1` feature can be promoted to `STAGE-2` at any time. Ideally a
+feature should stay in `STAGE-1` for about one year, but may stay longer if we
+don't see it used enough in the wild.
+
+**Promotion Checklist:**
+[] There is general consensus that the feature has been proven to be a good
+  addition to JSON Schema and is unlikely to change.
+[] We see the feature being used successfully in the wild and not generating a
+  lot of community support issues that could indicate a problem.
+
+### STAGE-2
+`STAGE-2` features are in the last stages of becoming stable. Changes are
+unlikely, but backward incompatible changes are still allowed in extreme cases.
+Users can be reasonably sure that the feature won't change, but can't expect all
+implementations to support the feature yet.
+
+`STAGE-2` is a staging area for features that are ready to be declared stable,
+but are waiting for the next release or don't yet have enough implementations.
+When we see the vast majority of actively maintained implementations support
+this feature, it will be promoted to stable in the next release. Implementers
+SHOULD support `STAGE-2` features so they aren't out-of-date when the next
+release happens.
+
+Being in `STAGE-2` isn't a guarantee that a feature will be declared stable in
+the next release. If we don't see that the vast majority of actively maintained
+implementation have implemented the feature, it may stay in `STAGE-2` another
+year to allow implementations to catch up.
+
+`STAGE-2` features can only be promoted to stable as part of a release.
+
+**Promotion Checklist:**
+[] The vast majority of actively maintained implementations support the feature
+
+### Stable
+Any part of the specification that doesn't have stability flag or has a year
+stability flag such as `2024` is considered stable. This flag indicates the year
+a feature was declared stable. Any changes to stable features MUST be backward
+compatible and not introduce problems for forward compatibility. Stable features
+are never removed, but may be deprecated.
+
+### Deprecated
+Any feature in the specification that is flagged in the form `DEPRECATED-{YEAR}`
+is considered deprecated as of the year specified. Implementations SHOULD still
+support these features to maintain backward compatibility. Deprecated features
+will never be removed from the spec, but schema authors SHOULD migrate away from
+using them as implementations may begin dropping support for these features at
+some point.
+
+A feature can only be deprecated as part of a release.
+
+## Release Process
+Releases occur on Jan 1 every year. Each release is identified with the
+four-digit year of when the release occurred. Releases never introduce anything
+new. The only change should be updating `STAGE-2` flags to `{YEAR}` flags for
+the features that are promoted to stable that year, or adding adding deprecation
+flags for any features that are deprecated that year.
+
+Implementations that express support for a particular release MUST support all
+features that are stable as of that release.
+
+A snapshot of the spec will be taken of the stable parts of the spec and made
+available as a PDF on the website at.
+
+## Meta-Schemas
+Meta-schemas associated with the specification(s) maintained under this process
+are subject to the same compatibility rules as the specification. They can be
+updated at any time, but changes MUST be backward compatible and allow for
+forward compatibility with future schemas.
+
+Meta-schema URIs SHOULD not change once published, but if they do, the old URI
+MUST remain available and redirect to the new URI.
+
+Meta-schemas are considered non-normative. It's sometimes not possible or not
+practical for the meta-schema to describe every constraint in the specification.