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
I guess for the operation of the seeds we have a limit on the retention time in place.

I started asking about statistics and aggregate data because like you I believe there is a use for some statistics, if handled carefully.

We need to define two granularity levels:
 - granularity for logging for operational purposes
 - granularity for information to be released to the general public for network health monitoring

What I gathered so far from the discussion is that any logging is ok as long as it is strictly needed for the operation of the seed (still a bit vague for my taste) and we have a limited retention time. For the granularity of data to be released to the public some are pushing for total silence.

I'm ok with these if everybody agrees, but the current formulation is ambiguous as it concentrates on logs of individual queries, which is why I started asking about aggregated data.