update CHANGELOG.md

Author: StijnCaerts

CHANGELOG.md

@@ -12,6 +12,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 - Added the ability to set timeout for Opensearch and Elasticsearch clients by setting the environmental variable `ES_TIMEOUT` [#408](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/408)
 
+### Changed
+
+- Updated collection to index logic to support searching a large amount of indices [#412](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/412)
+
 ## [v6.0.0] - 2025-06-22
 
 ### Added