Update README.IPv6 to match what is in 2.3.0

IPv6 is no longer provided by external patches - all has been
integrated.  Document that fact, point at the new configuration options,
and at potential caveats.

Acked-by: David Sommerseth <davids@redhat.com>
Message-Id: 1359113954-25768-1-git-send-email-gert@greenie.muc.de
URL: http://article.gmane.org/gmane.network.openvpn.devel/7305
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit c2f4c1918202f4395438663804b0806c93790501)