doc: note validation and timeline required for stables

doc: note validation and timeline required for stables

If a stable branch for a specific DPDK release is to proceed,
along with needing a maintainer, there should also be commitment
from major contributors for validation of the releases.

Also, as decided in the March 27th techboard, to facilitate user
planning, a release should be designated as a stable release
no later than 1 month after it's initial master release.

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