index.md: replace qwebirc link with kiwi button.

* SSL is used for connecting to freenode.
* No captcha.
* Looks nicer than some link.
* KiwiIRC looks nicer than qwebirc.

The button looks like this: [![Visit our IRC channel](https://kiwiirc.com/buttons/chat.freenode.net/%23feminist.png)](https://kiwiirc.com/client/chat.freenode.net:+6697/##feminist)

I think that my vimrc also replaced tabs with spaces or converted to Unix-line-endings or both or something as I only changed one line.