Documentation: add section about git to email-clients.txt

These days most people use git to send patches so I have added a section
about that.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>