maintainer-pgp-guide: add a section on PGP-signed patches

With more developers beginning to use b4 and patatt, add a section to
the guide that talks about setting up and using patatt for PGP-signing
patch submissions.

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Link: https://lore.kernel.org/r/20220727-docs-pgp-guide-v2-4-e3e6954affb6@linuxfoundation.org


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