Document DNS setup

[JeremyRand]

It would be useful for the README.md to document how DNS is supposed to be set up. My assumption here is that it looks something like the following:

1. Run the seeder on a machine whose IP is accessible via seeder-backend.example.org.
2. Put an NS record at seeder.example.org pointing to seeder-backend.example.org..
3. Use seeder.example.org as the DNS seed in a Bitcoin client.

But it took me a few minutes to figure out that this was the likely setup procedure, and I suspect it's non-obvious to a significant set of people.