Update README.md

adding in a note to advise against using latest, or not specifying a tag at all is against best practise

Author: davejrt

README.md

@@ -343,6 +343,8 @@ The `extra_parameters` parameter contains an array of command line arguments to
 
 By default, automatic restarting of the service on failure is enabled by the service file for systemd based systems.
 
+It is highly recommended that an image tag be used at all times with the `docker::run` define type. If not, the latest image will be used, whether it be in a remote registry or installed on the server already by the `docker::image` define type. 
+
 To use an image tag, add the following code to the manifest file:
 
 ```puppet