Add zsh completion for 'docker network create --attachable'

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

contrib/completion/zsh/_docker

@@ -1160,6 +1160,7 @@ __docker_network_subcommand() {
         (create)
             _arguments $(__docker_arguments) -A '-*' \
                 $opts_help \
+                "($help)--attachable[Enable manual container attachment]" \
                 "($help)*--aux-address[Auxiliary IPv4 or IPv6 addresses used by network driver]:key=IP: " \
                 "($help -d --driver)"{-d=,--driver=}"[Driver to manage the Network]:driver:(null host bridge overlay)" \
                 "($help)*--gateway=[IPv4 or IPv6 Gateway for the master subnet]:IP: " \