Adds documentation for additional groups.

Signed-off-by: Mrunal Patel <mrunalp@gmail.com>

docs/reference/commandline/run.md

@@ -34,6 +34,7 @@ weight=1
       --entrypoint=""            Overwrite the default ENTRYPOINT of the image
       --env-file=[]              Read in a file of environment variables
       --expose=[]                Expose a port or a range of ports
+      --group-add=[]             Add additional groups to run as
       -h, --hostname=""          Container host name
       --help=false               Print usage
       -i, --interactive=false    Keep STDIN open even if not attached