Updated v3 release notes (markdown)

v3-release-notes.md

@@ -158,6 +158,30 @@ user = User(name="Leia", _private_state="Extra info")
 assert user._private_state == "Extra info"
 ```
 
+### Graphene `Schema` no longer subclasses GraphQLSchema type
+
+The Graphene `Schema` type no longer subclasses the `GraphQLSchema` type but instead references it through the `graphql_schema` attribute.
+
+Before:
+
+```python
+from graphql import GraphQLSchema
+from graphene import Schema
+
+schema = Schema(query=Query)
+assert isinstance(schema, GraphQLSchema)
+```
+
+After:
+
+```python
+from graphql import GraphQLSchema
+from graphene import Schema
+
+schema = Schema(query=Query)
+assert isinstance(schema.graphql_schema, GraphQLSchema)
+```
+
 ---
 
 A huge thanks to everyone involved in bringing this release together!