README.md: update KiwiIRC button

* Use chat.freenode.net instead of irc.freenode.net
   * It's recommended by freenode everywhere https://freenode.net/irc_servers.shtml
* Use SSL and SSL port to connect to freenode.

I don't know is this the correct branch as the README.md only says that pull requests are welcome and there isn't CONTRIBUTING.md file.