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
Oh I see how you're reading this. In light of that it should say instead:

> their serving infrastructure and not sell or transfer control of their
> DNSseed.

The expectations here are largely not directly enforceable by technology (or we wouldn't need to ask for them, they'd just be enforced), so there is a degree of reliance on honest behavior by operators.  What I want to address here is the risk that some anonymous party comes up with a way to exploit the position of being a DNS seed for a dishonest end and they offer to buy control of a DNSseed from an existing operator (without mentioning their intended attack). An honest operator should, per these expectations (if not common sense first), refuse such a request.