Update README

Maybe clarify that you can call methods on objects like this:

```
expose :categories, :using => CategoryEntity do | instance |
    instance.method_that_returns_categories(option_1, option_2)
end
```

Useful if you use a gem like [ActsAsTaggableOn](https://github.com/mbleigh/acts-as-taggable-on) for tags, where you can't access the children by just calling `parent.child`.
