CONTRIBUTING.md: use proper language && fix links.

The links went to supybot.fr.cr where the website moved away long time ago.
README.md: Sanify topis & fix IRC links.

IRC links were going to irc.freenode.net which is CNAME for
chat.freenode.net .
Merge pull request #587 from Mkaysi/readme-contributing

Update CONTRIBUTING.md and README.md.