docs: add type_overrides feature to CHANGES.md

Author: enzomaruffa

CHANGES.md

@@ -67,6 +67,10 @@
 
 * Added DATE, DATETIME, and JSON type support to BigQuery I/O for use with Storage Write API and BEAM_ROW format (Python) ([#25946](https://github.com/apache/beam/issues/25946)).
 
+## New Features / Improvements
+
+* Added `type_overrides` parameter to `WriteToBigQuery` allowing users to override default BigQuery to Python type mappings when using Storage Write API (Python) ([#25946](https://github.com/apache/beam/issues/25946)).
+
 # [2.71.0] - Unreleased
 
 ## I/Os