update CHANGELOG

Author: StijnCaerts

CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Added
 
+- use index templates for Collection and Item indices [#208](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/discussions/208)
+
 ### Changed
 
 ### Fixed