add changelog

Signed-off-by: zhichao-aws <[email protected]>

CHANGELOG.md

@@ -10,6 +10,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Fix async actions are left in neural_sparse query ([#438](https://github.com/opensearch-project/neural-search/pull/438))
 - Fix typo for sparse encoding processor factory([#578](https://github.com/opensearch-project/neural-search/pull/578))
 - Add non-null check for queryBuilder in NeuralQueryEnricherProcessor ([#615](https://github.com/opensearch-project/neural-search/pull/615))
+- Add max_token_score field placeholder in NeuralSparseQueryBuilder to fix the rolling-upgrade from 2.x nodes bwc tests. ([#696](https://github.com/opensearch-project/neural-search/pull/696))
 ### Infrastructure
 ### Documentation
 ### Maintenance