release_notes: document more breaking changes

docs/release_notes.rst

@@ -20,9 +20,23 @@ Breaking changes:
     for a safe JSON-serializable representation.
 
 * Python 3.6 is no longer supported.
-* :meth:`Index.count <pynamodb.indexes.Index.count>`, :meth:`Index.query <pynamodb.indexes.Index.query>`,
-  and :meth:`Indexn.scan <pynamodb.indexes.Index.scan>` are now instance methods.
-* :py:class:`~pynamodb.settings.OperationSettings` has been removed
+* PynamoDB no longer has a default AWS region (used to be us-east-1) (:pr:`1003`).
+  If needed, update your models' `Meta` or set the `AWS_DEFAULT_REGION` environment variable.
+* :py:class:`~pynamodb.models.Model`'s JSON serialization helpers were changed:
+
+  * :code:`to_json` was renamed to :meth:`~pynamodb.attributes.AttributeContainer.to_simple_dict` (:pr:`1126`). Additionally, :meth:`~pynamodb.attributes.AttributeContainer.to_dynamodb_dict`
+    and :meth:`~pynamodb.attributes.AttributeContainer.from_dynamodb_dict` were added for round-trip JSON serialization.
+  * :code:`pynamodb.util.attribute_value_to_json` was removed (:pr:`1126`)
+
+* :py:class:`~pynamodb.attributes.Attribute`'s :code:`default` parameter must be either an immutable value
+  (of one of the built-in immutable types) or a callable.
+  This prevents a common class of errors caused by unintentionally mutating the default value.
+  A simple workaround is to pass an initializer (e.g. change :code:`default={}` to
+  :code:`default=dict`) or wrap in a lambda (e.g. change :code:`default={'foo': 'bar'}` to
+  :code:`default=lambda: {'foo': 'bar'}`).
+* :meth:`~pynamodb.indexes.Index.count`, :meth:`~pynamodb.indexes.Index.query`,
+  and :meth:`~pynamodb.indexes.Index.scan` are now instance methods.
+* :py:class:`~pynamodb.settings.OperationSettings` has been removed.
 
 Major changes: