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 rephrasing the first sentence to "Any logging of DNS queries, or derivatives thereof, must be limited to the scope necessary for the operation of the service." to explicitly include statistics and aggregations.

I find the part about urgent health of the network confusing and might create loopholes.