Coccinelle: Update documentation

 - Add information about use of the C={1,2} make flag
 - Add a description of the new chain mode mechanism
 - Add a link to the wiki

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