Changed add to group command to include -a (append) flag, for safety. In response to customer email.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)

docs/sources/installation/openSUSE.md

@@ -45,9 +45,9 @@ If we want Docker to start at boot, we should also:
 
 The docker package creates a new group named docker. Users, other than
 root user, need to be part of this group in order to interact with the
-Docker daemon.
+Docker daemon. You can add users with:
 
-    $ sudo usermod -G docker <username>
+    $ sudo usermod -a -G docker <username>
 
 **Done!**