Add Gitter chat badge to README

The [LiveScript Gitter chat](https://gitter.im/gkz/LiveScript) is mainly inactive, but I think it could be great if we started using it. Since it allows github login, it would make it much easier to find other LiveScripters, and could reduce the multitude of [gh issues](https://github.com/gkz/LiveScript/issues) that are about really small problems.

I'm aware of [/r/livescript](http://www.reddit.com/r/livescript/) and [the Google Group](https://groups.google.com/forum/#!forum/livescript), but they're better oriented for large topics, don't work with a github account, and aren't "officially" endorsed either as far as I can tell.

What do you think?