Document issue with --chroot, /dev/urandom and PolarSSL.

See trac#218

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <1389441036-12538-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8213
(cherry picked from commit b238a1f2d4b2cdcfc844689b33fd3ac43ed31c1c)