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
How about using a similar level of granularity as the EDNS Client Subnet extensions (http://tools.ietf.org/html/draft-vandergaast-edns-client-subnet-02). The originating IP address is truncated to the BGP prefix in logs used for statistical analysis. This would prevent any analysis from identifying an individual user, which is the rationale behind point 4 if I'm not mistaken and would still allow to do research based on the query logs.

As it stands right now, point 4 simply is too restrictive.