Be slightly more careful for beginners

Basic changes aimed at making life easier for people who first approach this.

I was wondering if I should add a line saying one can use Network-manager (on Linux) to share a WIFI internet connection with the Cubie using an Ethernet cable but have not done it.  Let me know if you think this would be useful.
It's worth noting here that we do install `avahi` by default in xen-arm-builder to broadcast its name (`$BOARD.local`, e.g. `cubieboard2.local`) instead of having to scan for the IP address.