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
Ack, sounds reasonable, thanks for clarifying. I would however allow counting queries by type (A, AAAA, SRV, ...) since at least in my case that helped debug quite a few issues with my DNS seed.