Update README

[filipstefansson]

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 for tags, where you can't access the children by just calling parent.child.

[dblock]

Please feel free to make a PR, appreciated.