Improve HoC connectToStores Docs

I think that the connectToStores HoC pattern is a huge plus for Flummox, and that the actual docs don't showcase how awesome it is (Easy, intuitive Relay-like data fetching from stores).

I'd be happy to send a PR with an updated Quick start, examples and a "Why HoC > FluxComponent > fluxMixin". Also I could introduce

```
contextTypes: {
flux: React.PropTypes.object
},
```

because without it HoC becomes very clunky.

@acdlite basically I just want to check if this is the direction that you want on the guide/docs?