linux-phytec-ti

Documentation/email-clients.txt: convert it to ReST markup

As this file is mentioned at the development-process/ book,
let's convert it to ReST markup.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>