[seeder] Add some useful information about testing in the README

Summary:
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.

Backport of my upstream change here: https://github.com/sipa/bitcoin-seeder/pull/96

Test Plan:

./bitcoin-seeder -host=dnsseed.example.com -ns=localhost -mbox=thonkle@protonmail.com -port=15353
dig @:: -p 15353 dnsseed.example.com

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10834