Define OpAMP Protobuf schema stability guarantees (#141)

The guarantees mostly mirror the proposal for OTLP
(open-telemetry/opentelemetry-proto#432), the primary
difference being that OpAMP has no gRPC services defined and does not need
any guarantees for service definitions.

Resolves #134

specification.md

@@ -154,6 +154,7 @@ Note: this document requires a simplification pass to reduce the scope, size and
   * [Interoperability of Future Capabilities](#interoperability-of-future-capabilities)
     + [Ignorable Capability Extensions](#ignorable-capability-extensions)
     + [Non-Ignorable Capability Extensions](#non-ignorable-capability-extensions)
+    + [Protobuf Schema Stability](#protobuf-schema-stability)
 - [Performance and Scale](#performance-and-scale)
 - [FAQ for Reviewers](#faq-for-reviewers)
   * [What is WebSocket?](#what-is-websocket)
@@ -2571,6 +2572,31 @@ peer and adjust their behavior appropriately. How exactly the behavior is
 adjusted for future capabilities MUST be defined in the future specification of
 the new capabilities.
 
+### Protobuf Schema Stability
+
+The specification provides the follow stability guarantees of the
+[Protobuf definitions](proto/opamp.proto) for OpAMP 1.0:
+
+- Field types, numbers and names will not change.
+- Names of messages and enums will not change.
+- Numbers assigned to enum choices will not change.
+- Names of enum choices will not change.
+- The location of messages and enums, i.e. whether they are declared at the top lexical
+  scope or nested inside another message will not change.
+- Package names and directory structure will not change.
+- `optional` and `repeated` declarators of existing fields will not change.
+- No existing symbol will be deleted.
+
+Future versions of the OpAMP specification may be extended by modifying the
+Protobuf schema defined in this specification version. The following Protobuf schema
+changes are allowed, provided that they comply with the interoperability requirements
+defined elsewhere in this specification:
+
+- Adding new fields to existing messages.
+- Adding new messages or enums.
+- Adding new choices to existing enums.
+- Adding new choices to existing oneof fields.
+
 # Performance and Scale
 
 TBD