Documentation: process: Document suitability of Proton Mail for kernel development

Documentation: process: Document suitability of Proton Mail for kernel development

Proton Mail automatically picks up PGP keys for those with kernel.org
accounts (and other domains!) which provide WKD for their users & uses
them to encrypt emails, including patches.

Document the behaviour & Proton Mail's unsuitability for kernel
development.

Reviewed-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20221231152320.1340874-1-conor@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>