Incomplete documentation around FluxComponent

I spent a good hour trying to figure this out until I found #61. `flux` it is not available on the react context unless you explicitly include it in `contextTypes`:

``` js
contextTypes: {
flux: React.PropTypes.any
}
```

`this.context.flux` is undefined in a react component until you add the `contextType`. Just adding a note to that effect to the docs would be enormously helpful to beginners.