docs: submitting-patches: describe preserving review/test tags

From time to time, the novice kernel contributors do not add Reviewed-by
or Tested-by tags to the next versions of the patches.  Mostly because
they are unaware that responsibility of adding these tags in next
version is on submitter, not maintainer.
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>

Link: https://lore.kernel.org/r/20201013162725.13572-1-krzk@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/process/submitting-patches.rst

@@ -527,6 +527,13 @@ done on the patch.  Reviewed-by: tags, when supplied by reviewers known to
 understand the subject area and to perform thorough reviews, will normally
 increase the likelihood of your patch getting into the kernel.
 
+Both Tested-by and Reviewed-by tags, once received on mailing list from tester
+or reviewer, should be added by author to the applicable patches when sending
+next versions.  However if the patch has changed substantially in following
+version, these tags might not be applicable anymore and thus should be removed.
+Usually removal of someone's Tested-by or Reviewed-by tags should be mentioned
+in the patch changelog (after the '---' separator).
+
 A Suggested-by: tag indicates that the patch idea is suggested by the person
 named and ensures credit to the person for the idea. Please note that this
 tag should not be added without the reporter's permission, especially if the