gerrit: add example usages for `reviewers` action

It wasn't really clear which way around the arguments should have been
(CL first or reviewer email(s) first), so let's add a couple of
examples.

BUG=None
TEST=ran `gerrit --help`, and used `gerrit reviewers` a few times

Change-Id: Id15d3321b0d126c9a09ca8fcb491f3794aa438ed
Reviewed-on: https://chromium-review.googlesource.com/1481976
Commit-Ready: Harry Cutts <hcutts@chromium.org>
Tested-by: Harry Cutts <hcutts@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/gerrit.py b/scripts/gerrit.py
index d689585..7640419 100644
--- a/scripts/gerrit.py
+++ b/scripts/gerrit.py
@@ -538,6 +538,10 @@
   $ gerrit inspect 28123    # Inspect CL 28123 on the public gerrit.
   $ gerrit inspect *28123   # Inspect CL 28123 on the internal gerrit.
   $ gerrit verify 28123 1   # Mark CL 28123 as verified (+1).
+  $ gerrit reviewers 28123 foo@chromium.org    # Add foo@ as a reviewer on CL \
+28123.
+  $ gerrit reviewers 28123 ~foo@chromium.org   # Remove foo@ as a reviewer on \
+CL 28123.
 Scripting:
   $ gerrit ready `gerrit --raw mine` 1      # Mark *ALL* of your public CLs \
 ready.