add change log

Author: kristenTian

CHANGELOG.md

@@ -20,6 +20,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 - [Multiple DataSource] Add support for SigV4 authentication ([#3058](https://github.com/opensearch-project/OpenSearch-Dashboards/pull/3058)). Backwards-compatible feature included in v2.6.0 release.
 - Add plugin manifest config to define OpenSearch plugin dependency and verify if it is installed on the cluster ([#3116](https://github.com/opensearch-project/OpenSearch-Dashboards/pull/3116))
 - Replace re2 with RegExp in timeline and add unit tests ([#3908](https://github.com/opensearch-project/OpenSearch-Dashboards/pull/3908))
+- [Multiple DataSource] Backend support for adding sample data ([#4268](https://github.com/opensearch-project/OpenSearch-Dashboards/pull/4268))
 
 ### 🐛 Bug Fixes