Add description of neural sparse two-phase enhancement in CHANGLOG.md.

Signed-off-by: conggguan <[email protected]>

CHANGELOG.md

@@ -21,6 +21,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - BWC tests for text chunking processor ([#661](https://github.com/opensearch-project/neural-search/pull/661))
 - Allowing execution of hybrid query on index alias with filters ([#670](https://github.com/opensearch-project/neural-search/pull/670))
 - Allowing query by raw tokens in neural_sparse query ([#693](https://github.com/opensearch-project/neural-search/pull/693))
+- Enhance neural_sparse query's latency performance with two-phase rescore query([#695](https://github.com/opensearch-project/neural-search/pull/695/files)).
 ### Bug Fixes
 - Add support for request_cache flag in hybrid query ([#663](https://github.com/opensearch-project/neural-search/pull/663))
 ### Infrastructure