update release notes (opensearch-project#654) (opensearch-project#659)

Signed-off-by: Surya Sashank Nistala <snistala@amazon.com>

Signed-off-by: Surya Sashank Nistala <snistala@amazon.com>
(cherry picked from commit f842a046f4970e81b215fd650da38cec784a21ab)

Co-authored-by: Surya Sashank Nistala <snistala@amazon.com>

release-notes/opensearch-alerting.release-notes-2.4.0.0.md

@@ -12,6 +12,7 @@ Compatible with OpenSearch 2.4.0
 * Custom history indices ([#616](https://github.com/opensearch-project/alerting/pull/616]))
 * adds filtering on owner field in search monitor action ([#641](https://github.com/opensearch-project/alerting/pull/641]))
 * Support to specify backend roles for monitors ([#635](https://github.com/opensearch-project/alerting/pull/635]))
+* Adds findings in bucket level monitor ([#636](https://github.com/opensearch-project/alerting/pull/636]))
 
 ### Refactoring
 * moving over data models from alerting to common-utils ([#556](https://github.com/opensearch-project/alerting/pull/556]))