clarify usage of --

runcron.c

@@ -29,7 +29,7 @@ char *lock_dir = "/var/tmp";
 int usage() {
   fprintf(stderr,
           "%s v%s - a cron job wrapper\n"
-          "Usage: %s [<OPTIONS>] <COMMAND> [<ARGS>...]\n"
+          "Usage: %s [<OPTIONS>] [--] <COMMAND> [<ARGS>...]\n"
           "\n"
           "Known options:\n"
           "\n"
@@ -59,6 +59,9 @@ int usage() {
           "                     lock to be released, failing with an error if after SECONDS\n"
           "                     the lock could not be obtained. If set to 0 (the default),\n"
           "                     wait forever. Specifying this option implies --wait.\n"
+          "\n"
+          "If you are passing options to COMMAND, remember to use `--' to tell the runcron\n"
+          "option parser to stop interpreting options for itself.\n"
           "\n",
           PACKAGE, PACKAGE_VERSION, PACKAGE
           );