Add policy for DNS seeds

@gmaxwell wrote down a few rules to make it clear what is expected of DNS seeds and their operators.

Rendered document: https://github.com/laanwj/bitcoin/blob/2014_07_dnsseed_policy/doc/dnsseed-policy.md
"Fairly selected" might be good to define, but maybe not very easy to. Some seed nodes may out of necessity only index IPv4 or IPv6, so discrimination based on IP/address in general can't be prohibited. OTOH, leaving this undefined may be better just to avoid loopholes around common sense...