docs: add note about using backticks for rbs in gitlab

So that gitlab will render the < and > correctly allowing the tag to be
copy-n-pasted without additional formatting.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

docs/submittingpatches.html

@@ -302,6 +302,17 @@ These Reviewed-by, Acked-by, and Tested-by tags should also be amended
 into commits in a MR before it is merged.
 </p>
 
+<p>
+When providing a Reviewed-by, Acked-by, or Tested-by tag in a gitlab MR,
+enclose the tag in backticks:
+</p>
+<pre>
+  `Reviewed-by: Joe Hacker &lt;jhacker@example.com&gt;`</pre>
+<p>
+This is the markdown format for literal, and will prevent gitlab from hiding
+the &lt; and &gt; symbols.
+</p>
+
 <p>
 Review by non-experts is encouraged.  Understanding how someone else
 goes about solving a problem is a great way to learn your way around