Add zsh completion for 'service {create,update} --group-{add,rm}'

Signed-off-by: Steve Durrheimer <s.durrheimer@gmail.com>

contrib/completion/zsh/_docker

@@ -1083,6 +1083,7 @@ __docker_service_subcommand() {
         "($help)*--constraint=[Placement constraints]:constraint: "
         "($help)--endpoint-mode=[Placement constraints]:mode:(dnsrr vip)"
         "($help)*"{-e=,--env=}"[Set environment variables]:env: "
+        "($help)*--group-add=[Add additional user groups to the container]:group:_groups"
         "($help)*--label=[Service labels]:label: "
         "($help)--limit-cpu=[Limit CPUs]:value: "
         "($help)--limit-memory=[Limit Memory]:value: "
@@ -1177,6 +1178,7 @@ __docker_service_subcommand() {
                 "($help)--arg=[Service command args]:arguments: _normal" \
                 "($help)*--container-label-add=[Add or update container labels]:label: " \
                 "($help)*--container-label-rm=[Remove a container label by its key]:label: " \
+                "($help)*--group-rm=[Remove previously added user groups from the container]:group:_groups" \
                 "($help)--image=[Service image tag]:image:__docker_repositories" \
                 "($help -)1:service:__docker_complete_services" && ret=0
             ;;