update readme..

It seems that unless you use USB tethering, you need to manually create
an /etc/resolv.conf in order to have working network access for the 'yum
install'..  easiest way to deal with that is make USB tethering a
required step.
start-chroot.sh should also use our own busybox
For now, use --nogpgcheck for install..

not really ideal.. but at the moment there seems to intermittenly be
unsigned packages in f19/arm repos.  Until that is sorted, use
--nogpgcheck :-(