Add hint for IPv6 only machines

[KastenKlicker]

The -Djava.net.preferIPv6Addresses=true flag must be present on IPv6 only machines when first starting a paper server or otherwise it will fail to download the vanilla server file.

This is a good hint for all newbies, which should be present in the paper Getting Started docs.

I can add some IPv6 docs myself as I'm setting up velocity and paper on an IPv6 only machine at the moment. If someone can tell me where to put them, I would be happy to help ;)