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
Hm, didn't know that. That is assuming that all nodes behave like Bitcoin Core though, if a node decides to send an addr on behalf of one of its peers then that will be forwarded, right? That would explain a strange behavior I had a few weeks ago, but that's probably off-topic, sorry :-)