Documentation/process: kernel maintainer PGP guide

This guide is an adapted version of the more general "Protecting Code
Integrity" guide written and maintained by The Linux Foundation IT for
use with open-source projects. It provides the oft-lacking guidance on
the following topics:

- how to properly protect one's PGP keys to minimize the risks of them
  being stolen and used maliciously to impersonate a kernel developer
- how to configure Git to properly use GnuPG
- when and how to use PGP with Git
- how to verify fellow Linux Kernel developer identities

I believe this document should live with the rest of the documentation
describing proper processes one should follow when participating in
kernel development. Placing it in a wiki on some place like kernel.org
would be insufficient for a number of reasons -- primarily, because only
a relatively small subset of maintainers have accounts on kernel.org,
but also because even those who do rarely remember that such wiki
exists. Keeping it with the rest of in-kernel docs should hopefully give
it more visibility, but also help keep it up-to-date as tools and
processes evolve.

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>