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
A crawler returns nothing on clients which are either not listening/advertising or on full nodes which are not advertising and are only listening to a subset of the network.  The document already makes an effort to separate data produced from crawling without the aid of a privileged position in the network.

Without reaching any conclusions on the merits of collecting data on users, what you're proposing is equivalent to an explicit phone-home feature but less transparent and less subject to user choice and privacy controls because it silently piggybacking on an existing infrastructure.

If it were desirable to have phone-home user monitoring it should be done as an explicit feature so that its privacy implications can be carefully considered and so that it could be disabled independently of the rest of the system. 

I had no idea that anyone would have thought it even remotely acceptable to utilize the privileged position of operating a DNS seed to track users, but the purpose of this document is to minimize those sorts of surprises.