Add changelog.md

Signed-off-by: Arpit Bandejiya <abandeji@amazon.com>

CHANGELOG.md

@@ -17,6 +17,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Views, simplify data access and manipulation by providing a virtual layer over one or more indices ([#11957](https://github.com/opensearch-project/OpenSearch/pull/11957))
 - Add Remote Store Migration Experimental flag and allow mixed mode clusters under same ([#11986](https://github.com/opensearch-project/OpenSearch/pull/11986))
 - [Admission Control] Integrate IO Usage Tracker to the Resource Usage Collector Service and Emit IO Usage Stats ([#11880](https://github.com/opensearch-project/OpenSearch/pull/11880))
+- Add cluster primary balance contraint for rebalancing with buffer ([#12656](https://github.com/opensearch-project/OpenSearch/pull/12656))
 
 ### Dependencies
 - Bump `log4j-core` from 2.18.0 to 2.19.0