Add instructions on how to get help on commands

Signed-off-by: Ben Firshman <ben@firshman.co.uk>

docker/flags.go

@@ -88,6 +88,7 @@ func init() {
 		} {
 			help += fmt.Sprintf("    %-10.10s%s\n", command[0], command[1])
 		}
+		help += "\nRun 'docker COMMAND --help' for more information on a command."
 		fmt.Fprintf(os.Stderr, "%s\n", help)
 	}
 }