Add more SASL details to README
Add more SASL details to README

It took me a while to setup SASL on my machine, mostly because of
non-installed SASL modules. I expanded README a little bit to
note this.

Signed-off-by: Christophe Fergeau <05b95054a663c388a4e33d95828cb30f0370f8ef@redhat.com>

libvirt/libvirt-go,libvirt/libvirt-go,libvirt/libvirt-go,libvirt/libvirt-go