SubmittingPatches: Clarify requirements for patches directed toward stable

I misunderstood this section as simply asking me to add an email Cc, and
was unaware of the existence of Documentation/stable_kernel_rules.txt.
Hopefully this clarification will help save maintainers and new/rare
submitters time in the future.

Signed-off-by: Luke Dashjr <luke-jr+git@utopios.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>