Added changelog section

Author: mblaschke

documentation/UPDATE.md

@@ -9,6 +9,8 @@ If you have cloned the git repository it's safe to pull the new version.
 If you have used a zip file make sure to replace the `docker/` directory and also
 update `docker-compose.yml` and `docker-env.yml`.
 
+For [changes see CHANGELOG.md](/CHANGELOG.md)
+
 ## Update to 3.4.x
 As `PHP_UID` was replaced with `EFFECTIVE_UID` and `PHP_GID` was replaced with `EFFECTIVE_GID` you
 have to rebuild all containers with this variables: `main` and `web`