gerrit: add a tip for `gerrit help`

This will help people discover the help command more when using
`gerrit --help`.

BUG=None
TEST=ran `gerrit --help`

Change-Id: Ie2f919461cb1461df7d5b22fb1e26b9bf5a383f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3584274
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Lizzy Presland <zland@google.com>
Commit-Queue: Lizzy Presland <zland@google.com>
diff --git a/scripts/gerrit.py b/scripts/gerrit.py
index 5552b2f..b00d719 100644
--- a/scripts/gerrit.py
+++ b/scripts/gerrit.py
@@ -1172,7 +1172,8 @@
 
   site_params = config_lib.GetSiteParams()
   parser = commandline.ArgumentParser(
-      description=description, default_log_level='notice')
+      description=description, default_log_level='notice',
+      epilog='For subcommand help, use `gerrit help <command>`.')
 
   group = parser.add_argument_group('Server options')
   group.add_argument('-i', '--internal', dest='gob', action='store_const',