update CHANGELOG.md

Author: davidjgoss

CHANGELOG.md

@@ -6,6 +6,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
+### Changed
+- [JavaScript] BREAKING CHANGE: Emit types instead of classes ([#287](https://github.com/cucumber/messages/pull/287)) 
+
 ### Fixed
 - [python] Add a LICENSE file for Python ([#278](https://github.com/cucumber/messages/pull/278))
 - [.NET] Fix NuGet package generation