Add some useful information about testing in the README

[thonkle]

I found this to be helpful. Not being a DNS expert, I couldn't be sure if the seeder was not producing output or if my DNS records were broken or if my iptables were configured incorrectly. My confidence has improved now that I know how to eliminate the first case consistently. I think others will find this useful too.

[sipa]

ACK 929882b761b8a67bfca263c0ec7471f99940020e