doc: about email clients for Linux patches

Requested by Jeff Garzik.
v3, updated from lkml comments.

Add info about various email clients and their applicability
in being used to send Linux kernel patches.

Some notes takes from http://mbligh.org/linuxdocs/Email/Clients
Portions used with permission.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Jeff Garzik <jeff@garzik.org>
Cc: Martin Bligh <mbligh@mbligh.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>