Documentation: include sign off for reverts

Currently, we do not have any documentation on commit reverts regarding
the requirement of Signed-off-by tag for it. This may be misleading to
the users.

Evaluating MISSING_SIGN_OFF checkpatch warnings on v4.13..v5.8 showed
that 4 out of 11 cases missing a sign-off are revert commits.

Add documentation regarding the same to document the community
consensus and let readers know.
Signed-off-by: Aditya Srivastava <yashsri421@gmail.com>
Link: https://lore.kernel.org/r/20201110174749.32068-1-yashsri421@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/process/submitting-patches.rst

@@ -404,6 +404,8 @@ then you just add a line saying::
 
 using your real name (sorry, no pseudonyms or anonymous contributions.)
 This will be done for you automatically if you use ``git commit -s``.
+Reverts should also include "Signed-off-by". ``git revert -s`` does that
+for you.
 
 Some people also put extra tags at the end.  They'll just be ignored for
 now, but you can do this to mark internal company procedures or just