Filled out br0 config in README.md instructions

Added some more config info to br0 setup in `/etc/network/interfaces' file for those that cut and paste verbatim (guilty here). Failure to include broadcast, gateway, and network broke internet on the host. Now br0 is fully functional when host is rebooted.
Merge pull request #2 from brianclements/master

Filled out br0 config in README.md instructions