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
Just to be clear: this section is about logging of received DNS queries, not about crawling or data gathered through crawling.

For that purpose, DNS seeds are in a privileged position, as they get information from who is running bitcoin nodes, rather than who is running a reachable full node.