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
Aggregates that are OK: total number of queries, bandwidth up/down, CPU load, I/O load - these don't discriminate clients in any way, and require no logging of possibly identifying (meta)data.

Aggregates that are not OK: counting requests per country, client OS/program, number of unique IPs - these require acting on request contents or metadata.