Added PR details to CHANGELOG.md

Signed-off-by: Harsh Kothari <[email protected]>

Author: Harsh Kothari

CHANGELOG.md

@@ -16,6 +16,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Enabled Async Shard Batch Fetch by default ([#18139](https://github.com/opensearch-project/OpenSearch/pull/18139))
 - Allow to get the search request from the QueryCoordinatorContext ([#17818](https://github.com/opensearch-project/OpenSearch/pull/17818))
 - Improve sort-query performance by retaining the default `totalHitsThreshold` for approximated `match_all` queries ([#18189](https://github.com/opensearch-project/OpenSearch/pull/18189))
+- Added Auto Force Merge Manager to enhance hot to warm migration experience ([#18229](https://github.com/opensearch-project/OpenSearch/pull/18229))
 
 ### Changed