Add changelog entry for compatibility updates

Signed-off-by: xil <fridalu66@gmail.com>

Author: lucy66hw

CHANGELOG.md

@@ -10,6 +10,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 - Add PIT RPCs to `SearchService` ([#469](https://github.com/opensearch-project/opensearch-protobufs/pull/469)).
 
 ### Changed
+- Improve protobuf compatibility handling for optional and `oneof` field migrations ([#479](https://github.com/opensearch-project/opensearch-protobufs/pull/479)).
 
 ### Removed