Clarify when to include extensions in stac_extensions

extensions/README.md

@@ -2,6 +2,7 @@
 
 - [Overview](#overview)
 - [General Conventions](#general-conventions)
+- [The `stac_extensions` property](#the-stac_extensions-property)
 - [Stable STAC Extensions](#stable-stac-extensions)
 - [Community Extensions](#community-extensions)
   - [Extension Maturity](#extension-maturity)
@@ -43,6 +44,25 @@ Each extension has at least one *owner*. You can find extension owners in each e
 3. Additional attributes relating to a [Catalog](../catalog-spec/catalog-spec.md) or
    [Collection](../collection-spec/collection-spec.md) should be added to the root of the object.
 
+## The `stac_extensions` property
+
+[Catalog](../catalog-spec/catalog-spec.md#stac_extensions), [Collection](../collection-spec/collection-spec.md#stac_extensions) and
+[Item](../item-spec/item-spec.md#stac_extensions) objects have a `stac_extensions` property which contain URLs to JSON Schemas used to
+validate the implementation of an extension. These JSON Schema URLs act as identifiers for specific version of the extension that the
+object implements. The logic for when an object should list an extension JSON Schema URL (referred to as the extension ID) in it's
+`stac_extension` property is as follows:
+
+- If the object directly implements the extension, the  `stac_extensions` property of that object should contain the extension ID.
+- If an Asset implements properties of the extension, then `stac_extensions` property of the Item or Collection which holds that
+  Asset should contain the extension ID.
+- If a Collection [summary](../collection-spec/collection-spec.md#summaries) contains Item properties that implement an extension, then
+  the `stac_extensions` property of that Collection should list the extension ID. For example, if a Collection `summaries` property
+  contains a summary of `eo:bands`, then that Collection should have the EO extension JSON Schema URL in the `stac_extensions` property.
+- If an object implements an extension that results in properties from a separate extension to be referenced, then the latter extension
+  ID should be included in the `stac_extension` property for that object. For example, if a Collection implements the
+  [item_assets](https://github.com/stac-extensions/item-assets) extension, and in the `item_assets` property there is an Asset Definition
+  which includes `eo:bands`, then the EO extension ID should be listed in that Collection's `stac_extensions` property.
+
 ## Stable STAC Extensions
 
 These extensions are considered stable and are widely used in many production implementations. As additional extensions advance