Updated update.md

Author: mblaschke

documentation/UPDATE.md

@@ -2,7 +2,14 @@
 
 # Update
 
-# Update to 3.4.x
+## Preparation
+
+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`.
+
+## 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`