Added a statement about the compatibility of GPLv2 and WDK code

Signed-off-by: Samuli Seppänen <samuli.seppanen@gmail.com>
Acked-by: James Yonan <james@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Acked-by: David Sommerseth <dazo@users.sourceforge.net>
Acked-by: Alon Bar-Lev <alon.barlev@gmail.com>