Mention webpack#401 in changelog

Author: valscion

CHANGELOG.md

@@ -16,6 +16,7 @@ _Note: Gaps between patch versions are faulty, broken or test releases._
 
  * **Improvement**
    * Replace express with builtin node server, reducing number of dependencies ([#398](https://github.com/webpack-contrib/webpack-bundle-analyzer/pull/398) by [@TrySound](https://github.com/TrySound))
+   * Move `filesize` to dev dependencies, reducing number of dependencies ([#401](https://github.com/webpack-contrib/webpack-bundle-analyzer/pull/401) by [@realityking](https://github.com/realityking))
 
 ## 4.2.0