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
The purpose of the DNSseed infrastructure is to facilitate fast introduction to the network for hosts. Can you explain how (4) is at odds with this goal?

Identifying organizations which are using Bitcoin may still be harmful for those users and a violation of their privacy.