added details to multiple models example

I added some details in the README for the multiple models example.  I wasn't sure how this worked at first, and had to read the code to find out, so I figured some clarification might help.

On a tangent, have you thought about a syntax like this?

```javascript
var CommentView = React.createBackboneClass({
    expectModels: {
        user: "change:name",
        comment: null
        modelName: {opt1: 'bar', opt2: 'baz'} // maybe?  if additional params are needed
    }
    //, ...
});
```

Internally it could still wire up the mixins.  I don't know if it fits your style/design goals, but just figured I would throw it out there.

-Brandon