add change log

Author: kkewwei

CHANGELOG.md

@@ -32,6 +32,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Add ability for Boolean and date field queries to run when only doc_values are enabled ([#11650](https://github.com/opensearch-project/OpenSearch/pull/11650))
 - Refactor implementations of query phase searcher, allow QueryCollectorContext to have zero collectors ([#13481](https://github.com/opensearch-project/OpenSearch/pull/13481))
 - Adds support to inject telemetry instances to plugins ([#13636](https://github.com/opensearch-project/OpenSearch/pull/13636))
+- Add parameters to flat_object type ([#13853](https://github.com/opensearch-project/OpenSearch/pull/13853))
 
 ### Deprecated