SubmitChecklist: interfaces changes should CC linux-api@

Mention that patches that change the kernel-userland interface should
be CCed to the new list linux-api@vger.kernel.org.
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/SubmitChecklist

@@ -67,6 +67,8 @@ kernel patches.
 
 19: All new userspace interfaces are documented in Documentation/ABI/.
     See Documentation/ABI/README for more information.
+    Patches that change userspace interfaces should be CCed to
+    linux-api@vger.kernel.org.
 
 20: Check that it all passes `make headers_check'.