Add changelog entry for #230

Author: mattt

Changelog.md

@@ -9,6 +9,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- Added support for generating documentation for 
+  extensions to external types.
+  #230 by @Lukas-Stuehrk and @mattt. 
 - Added end-to-end tests for command-line interface.
   #199 by @MaxDesiatov and @mattt.
 - Added `--minimum-access-level` option to `generate` and `coverage` commands.