[FWaaS] Add note about deprecation of the neutron-fwaas project
[FWaaS] Add note about deprecation of the neutron-fwaas project

As we agreed in the drivers team, we will keep FWaaS API ref as official
API in the neutron-lib but we will not have any official implementation
of this API.
This patch adds such information to the FWaaS API ref document.

Change-Id: I7e683f52f47bbb7ce7c598c98332c5dbb5d97643

openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib