[WIP] - Create the documentation for the integration of Simple Form and Bootstrap.

This PR aims to create a guide showing how this application was built, using Simple Form and Bootstrap.
The documentation page looks like this right now:

![example application with simpleform and twitter bootstrap](https://cloud.githubusercontent.com/assets/611891/2674010/58030572-c0fa-11e3-98bb-1804b708b249.png)

Tasks:
- [x] Create the basic structure to the documentation page.
- [x] Add the basic documentation for configuring a new Rails app with Simple Form and Bootstrap.
- [x] Example to build a simple form using bootstrap.
- [x] Explain what `wrapper_mappings` does.
- [x] Improve the documentation style.