Add missing documentation for enabling federation auth plugin

The saml2 method and plugin must be declared in keystone.conf for
federation to be completely enabled.

Change-Id: Id68ffdc8a84d093406c836ee6988bfdb9e58d50f
Closes-Bug: #1290582
Co-Authored-By: Florent Flament <florent.flament-ext@cloudwatt.com>