Add BaseSerializer heading

docs/topics/3.0-announcement.md

@@ -30,6 +30,10 @@ See the [Version 3.0 GitHub issue](https://github.com/tomchristie/django-rest-fr
 
 **TODO**: Drop`.object`, use `.validated_data` or get the instance with `.save()`.
 
+#### The `BaseSerializer` class.
+
+**TODO**
+
 #### Always use `fields`, not `exclude`.
 
 The `exclude` option is no longer available. You should use the more explicit `fields` option instead.