mention atlas config in compose.yaml

Author: jordanstephens

samples/golang-mongodb-atlas/compose.yaml

@@ -9,6 +9,10 @@ services:
         target: 8080
         published: 8080
     environment:
+      # If you want to use MongoDB Atlas, you can set the URI with `defang config set MONGO_URI`
+      # and remove the value from the MONGO_URI environment variable so that it is read from defang
+      # config. For example:
+      # - MONGO_URI
       - MONGO_URI=mongodb://$MONGO_INITDB_ROOT_USERNAME:$MONGO_INITDB_ROOT_PASSWORD@db:27017/
     #deploy:
     #  resources: