docs: Add an encouraging note about providing reviews and acks.

Across several projects I've seen new contributors say "I wasn't sure if I
should provide a review tag since I'm not really an expert in this area."
Everyone I know already applies some implicit weighting to reviews from
different people, so encourage participation.

Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

docs/submittingpatches.html

@@ -302,6 +302,14 @@ These Reviewed-by, Acked-by, and Tested-by tags should also be amended
 into commits in a MR before it is merged.
 </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
+the project.  The submitter is expected to evaluate whether they have
+an appropriate amount of review feedback from people who also
+understand the code before merging their patches.
+</p>
+
 <h2 id="nominations">Nominating a commit for a stable branch</h2>
 
 <p>