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 intention is to absolutely prohibit using this as a privileged position to monitor users. "Aggregates" have a long history of surprising outcomes, especially because the user's threat model might not be what the aggregator is thinking about.

If we wanted to enable user-monitoring we would do so with a separate service specifically designed and intended for that so it could be transparent about its operation, so that the risks could be maximally mitigated, and so that users could opt out of it without otherwise degrading the operation of the software.

"Necessary for the operation" was intended to cover things like measuring traffic levels for capacity planning or investigating high load (e.g. to figure out how to block a DOS attack), or dumping queries that fail for software troubleshooting. Since it seems to be enabling some confusion here I'll think up some other language.