Mention the default input type for text columns and how to override with as: :string.

This information wasn't in the Readme and has caused me [confusion](http://stackoverflow.com/questions/9660432/simple-form-text-input) before when I didn't know what the valid options were for the `as:` option.

It would be nice if a complete list or table of `as:` options (also showing the default for each column type) were in the Readme ... or at least on a wiki page.