README: Use dnsmasq constructor for dhcp-range
README: Use dnsmasq constructor for dhcp-range

Since we're only using DHCPv6 to pass in some additional options, not to
assign addresses from, we can use a simple `constructor` with the interface
instead. This is nice since if the tunnel ever changes you don't need to
update the `dhcp-range` configuration in dnsmasq.

daenney/sixrd,daenney/sixrd