Documentation: Document how to get links with git am

This adds Kees' clever apply hook to the kernel documentation so
it can be easily references when needed.

Cc: Kees Cook <keescook@chromium.org>
Link: https://lists.linuxfoundation.org/pipermail/ksummit-discuss/2019-July/006608.html


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20191118223019.81708-1-linus.walleij@linaro.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>