CodingStyle: add information about editor modelines

I recently received a patch including a file that had a vim modeline,
and I realized that nothing specifically proscribed that practice.

Signed-off-by: Josh Triplett <josh@freedesktop.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>