minor documentation request

Can you please add a short subsection under Usage (in the top-level documentation README.md) for usage within a browser? The README explains how to _install_ it via

```

```

but then there are no subsequent instructions in the usage section for _using it_ (in a browser).

As far as I've been able to figure out, loading the .js file sets the global variable PEGjsCoffeePlugin, so that I can use it in PEG.js by adding the following option to the PEG.buildParser call:

```
plugins: [PEGjsCoffeePlugin]
```

But the README does not mention the name "PEGjsCoffeePlugin", so this was not easy to figure out.

thanks, -Neal