[PATCH] submit checklist: mention headers_check

Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

Documentation/SubmitChecklist

@@ -64,3 +64,5 @@ kernel patches.
 
 19: All new userspace interfaces are documented in Documentation/ABI/.
     See Documentation/ABI/README for more information.
+
+20: Check that it all passes `make headers_check'.