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
Yeah that's my fault. I find 'DNSseed' difficult to read, and changed it in the initial sentence, then saw it was used all over the place and forgot to change it back here.