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
While I do agree on the main purpose of the DNSSeed infrastructure, I do also believe that valuable insights can be gained from analyzing the queries, that might ultimately benefit Bitcoin itself.

I'm proposing the BGP prefix as a lower bound on the granularity of the collected data, which I believe is far more reasonable than an outright ban on any collection and use of this data. Data that by the way is still available to anyone with enough time to build a crawler of the network.