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
It's very easy to mess up anonymous statistics and leak information.  Can you suggest a way that would be better defined?  E.g. what level of statistics you think would be sensible? I don't see any big issue with raw traffic amounts.