docs: clarify docker-compose usage

Author: FinalAngel

docker-compose.yml

@@ -1,3 +1,6 @@
+# The docker-compose.yml file is __only__ used for local development.
+# This means that changes to this file will not affect cloud deployments in any way.
+# Read more at https://docs.divio.com/reference/docker-docker-compose/
 services:
   web:
     build: .