Don't turn off bridge iptables. NB:

 - this doesn't work anyway
   https://bugs.launchpad.net/ubuntu/+source/procps/+bug/50093
   a sensible workaround would be to set this in if-pre-up (see iptables)
 - I benched it a little and didn't notice anything startlingly different.
update readme for bridges & removing default network
enable ip_forward
add iptables rules