add xbill.png for doc
add xbill.png for doc

thierrymarianne/valuenetwork,valnet/valuenetwork,valnet/valuenetwork,FreedomCoop/valuenetwork,django-rea/nrp,FreedomCoop/valuenetwork,django-rea/nrp,django-rea/nrp,simontegg/valuenetwork,thierrymarianne/valuenetwork,simontegg/valuenetwork,FreedomCoop/valuenetwork,simontegg/valuenetwork,valnet/valuenetwork,simontegg/valuenetwork,thierrymarianne/valuenetwork,valnet/valuenetwork,django-rea/nrp,thierrymarianne/valuenetwork,FreedomCoop/valuenetwork