coccinelle.txt: update documentation to include M= option

Adding documentation for the new M= option which limits Coccinelle
to a specific set of directories.

Signed-off-by: Greg Dietsche <Gregory.Dietsche@cuw.edu>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Michal Marek <mmarek@suse.cz>