doc: advise to specify LTS branch when backporting patches

doc: advise to specify LTS branch when backporting patches

We have many stable branches being maintained at the same time, and
sometimes it's not clear which branch a patch is being backported for.
Note in the guidelines that it should be specified via the cover letter,
annotation or using --subject-prefix.
Also note to send only to stable@dpdk.org, not dev@dpdk.org.

Signed-off-by: Luca Boccassi <bluca@debian.org>
Acked-by: Thomas Monjalon <thomas@monjalon.net>