contrib/git-hooks: add a sendemail-validate example hook that adds FROM: lines to outgoing patch emails

This is useful for people using Microsoft Exchange / Office 360, which
butchers patches causing author identity to be lost.

(From OE-Core rev: 8dc690fddc1bace6b7af3f5e8af60c1625b067db)

Signed-off-by: Chris Laplante <chris.laplante@agilent.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>