add changelog

Author: lucy66hw

CHANGELOG.md

@@ -4,9 +4,10 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 
 ## [Unreleased]
 ### Added
-- Preprocessing: change prefix from underscore_ to x_ and require map key ([#216](https://github.com/opensearch-project/opensearch-protobufs/pull/216))
+- Preprocessing: Change prefix from underscore_ to x_ and require map key ([#216](https://github.com/opensearch-project/opensearch-protobufs/pull/216))
 - Add Cardinality and Missing aggregations. ([#245](https://github.com/opensearch-project/opensearch-protobufs/pull/245))
 - Add terms aggregation protos  ([#268](https://github.com/opensearch-project/opensearch-protobufs/pull/268))
+- Preprocessing: Handle unnamed additionalProperties.([#272](https://github.com/opensearch-project/opensearch-protobufs/pull/272))
 
 ### Changed
 - Update preprocessing for x-protobuf-excluded ([#266](https://github.com/opensearch-project/opensearch-protobufs/pull/266))