readme: fix mailing list and irc links

◦ list.perldancer.org doesn't work, so I swapped it with the mailing list link used on perldancer.org
◦ GitHub markdown doesn't support irc links, so I swapped it with the irc page on perldancer.org
Merge pull request #568 from patch/pr/readme-links

Fix mailing list and irc links in README.