Presenter docs could use an example

[softwaregravy]

I'm new to using a presenter pattern. I wasn't quite sure what to put in there.

I'm working off http://quickleft.com/blog/presenters-as-a-solution-to-asjson-woes-in-rails-apis as an example, but would be great to improve the docs in the gem.

[bploetz]

Pull requests welcome.