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
Ah, so the goal is to stop selling/leasing of the DNS seed by itself, but doing so as part of a larger transfer (company sale) is okay? I can't think of a good way to phrase this better :(