Document Yesod.Form.Bootstrap3

I felt like the Yesod.Form.Boostrap3 module could use some documentation based on my own experiences, [a question on the mailing list](https://groups.google.com/forum/#!topic/yesodweb/q9f52hSz31o), and a [StackOverflow question](http://stackoverflow.com/q/23350621/1176156). 

Preview available here http://maxgabriel.github.io/yesod/Yesod.Form.Bootstrap3.html (Though I feel like there must be a better way to share HTML/CSS/JS than uploading it to a Github blog...)

Current Haddocks for this module are [here](https://hackage.haskell.org/package/yesod-form/docs/Yesod-Form-Bootstrap3.html).

I haven't done much major Haddock work so I'm unsure how good my documentation is stylistically (for example, `$example` at the top of the file to reference paragraphs at the bottom—I copied that from Aeson).