docs: Add a note that MRs should still include any r-b or a-b tags.

v2: Mention "Tested-by" too

Reviewed-by: Dylan Baker <dylan@pnwbakers.com> (v1)
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

docs/submittingpatches.html

@@ -297,6 +297,10 @@ which tells the patch author that the patch can be committed, as long
 as the issues are resolved first.
 </p>
 
+<p>
+These Reviewed-by, Acked-by, and Tested-by tags should also be amended
+into commits in a MR before it is merged.
+</p>
 
 <h2 id="nominations">Nominating a commit for a stable branch</h2>