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
Just a point on the behavior of the software,

>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.

It will be forwarded if the node announces itself. If the node does not announce, there will be nothing to forward.  Setting listen=0, for example, disables announcements... or just being a non-full node client will also result in no announcements.