doc: prepare security process for vulnerabilities

doc: prepare security process for vulnerabilities

In case a vulnerability is discovered, the process to follow
is described in this document.
It has been inspired by the process of some referenced projects
and with the help of experts from Intel, RedHat, Mellanox
and the Linux Foundation.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Luca Boccassi <bluca@debian.org>