Clearly state where the fields can be used #21 (#23)

README.md

@@ -16,7 +16,14 @@ This is the place to add a short introduction.
 - [JSON Schema](json-schema/schema.json)
 - [Changelog](./CHANGELOG.md)
 
-## Item Properties and Collection Fields
+## Fields
+
+The fields in the table below can be used in these parts of STAC documents:
+- [ ] Catalogs
+- [x] Collections
+- [x] Item Properties (incl. Summaries in Collections)
+- [x] Assets (for both Collections and Items, incl. Item Asset Definitions in Collections)
+- [ ] Links
 
 | Field Name           | Type                      | Description |
 | -------------------- | ------------------------- | ----------- |