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
I would think a clause prohibiting sale of the server may be problematic to any organisationally-hosted seeds, even aside from hosting companies. What is the goal of prohibiting such transfers (without knowing, I can't come up with any alternative ideas)?