Add Documentation/coccinelle.txt

The purpose of this file is to document how to use Coccinelle and its
spatch tool to check the Linux kernel.

It gives information on where and how to retrieve Coccinelle, and how
to use it with the Coccinelle scripts integrated in the Linux kernel.

Signed-off-by: Nicolas Palix <npalix@diku.dk>
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Michal Marek <mmarek@suse.cz>