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 address of a node will be forwarded independently of whether it is listening to the network or not, you can collect the IP addresses of all nodes simply by listening for incoming addr messages.

I am trying to make it possible to make anonymized information available to the public in a controlled fashion in order to level the playing field. It is likely that some people will make use of the information they collect, ignoring the policy because it cannot be enforced by technical means. Without an agreed upon way to make this information available we actually increase the disparity between seed providers that have the privileged access and the rest that do not.

That being said, my seed will adhere to the policy, should the other seed providers agree to do so. I would however prefer if a fair-use policy were put in place that allows anonymized data to be used for research.