Update README to refelect the new binding behavior

[aliemjay]

For convenience reasons, miniserve will try to bind on all interfaces by default (if no -i is provided). It will also do that if explicitly provided with -i 0.0.0.0 or -i ::. In all of the aforementioned cases, it will bind on both IPv4 and IPv6.

Now -i 0.0.0.0 will bin to IPv4 only, and -i :: to IPv6.

[svenstaro]

@aliemjay Wanna write a short bit about this on the README?