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
> on behalf of one of its peers then that will be forwarded, right?

It will be, but thats broken. It isn't how the protocol works, any implementation that was doing that would be broken and it would be mildly harmful to the network. Of course, nodes can do malicious things and the system is generally robust against them.