Update CHANGELOG with rails-api#1645 and breaking change information

Author: remear

CHANGELOG.md

@@ -2,7 +2,11 @@
 
 Breaking changes:
 
+- [#1645](https://github.com/rails-api/active_model_serializers/pull/1645) Expanded KeyTransform into Transform. Changed :dashed transform to :dash. (@remear)
+- [#1574](https://github.com/rails-api/active_model_serializers/pull/1574) Default transform for the JsonApi adapter is :dashed. (@remear)
+
 Features:
+- [#1645](https://github.com/rails-api/active_model_serializers/pull/1645) Transforms (@remear)
 - [#1637](https://github.com/rails-api/active_model_serializers/pull/1637) Make references to 'ActionController::Base.cache_store' explicit
   in order to avoid issues when application controllers inherit from 'ActionController::API'. (@ncuesta)
 - [#1633](https://github.com/rails-api/active_model_serializers/pull/1633) Yield 'serializer' to serializer association blocks. (@bf4)