added image argument for private images

README.md

@@ -13,6 +13,10 @@ This is meant to be used as a [Drone plugin](http://plugins.drone.io/).
 - **deploy_key**: SSH private key part for ssh public key authentication
 - **stack**: Name of the stack to be deployed
 
+### Required (If Private Repo)
+
+- **image**: Name and Tag of image since private image's are not pulled automatically by docker stack deploy
+
 ### Optional (with defaults)
 
 - **compose** (default: `compose.yml`): compose file to use for deploying