update changelog

Author: jonhealy1

CHANGELOG.md

@@ -20,6 +20,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Elasticsearch index mappings updated to be more thorough.
 - Endpoints that return items (e.g., /search) now sort the results by 'properties.datetime,id,collection'.
   Previously, there was no sort order defined.
+- Db_to_stac serializer moved to core.py for consistency as it existed in both core and database_logic previously. 
+- Use genexp in execute_search and get_all_collections to return results.
+- Added db_to_stac serializer to item_collection method in core.py.
 
 ### Removed