Change log for configurable merge policy

Signed-off-by: Rishabh Maurya <rishabhmaurya05@gmail.com>

CHANGELOG.md

@@ -11,6 +11,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Add events correlation engine plugin ([#6854](https://github.com/opensearch-project/OpenSearch/issues/6854))
 - Introduce new dynamic cluster setting to control slice computation for concurrent segment search ([#9107](https://github.com/opensearch-project/OpenSearch/pull/9107))
 - Implement on behalf of token passing for extensions ([#8679](https://github.com/opensearch-project/OpenSearch/pull/8679))
+- Configurable merge policy for datastreams with option to choose between LogByteSize and Tiered merge policy ([#9992](https://github.com/opensearch-project/OpenSearch/pull/9992))
 
 ### Dependencies
 - Bump `log4j-core` from 2.18.0 to 2.19.0