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.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>