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
Yea, sorry— thats not the intent.  I mentioned hosting services at all because the first sentence of 0 could have been read as precluding the use of hosting services. Can you suggest a rephrase which is consistent with your own expectations?