Add missing usage()

bin/saturnin-push

 
 ORDER="$(saturnin-ini -p push.options.order)"
 
+usage() {
+    usage_is "[-o|--order path:host|host:path] host..."
+}
+
 while true; do case $1 in
     -o|--order)     ORDER=$2;           shift 2;    ;;
     -h|--help)      usage;                          ;;