Add doc reg. attributes with data prefix will be rendered.

There is the following undocumented behaviour that is works well with bootstrap, which would be nice to have it right in the readme.

``` clj
(f/render-field {:name :my-text :data-x 10 :data-y 20})
;; (nil [:input {:type "text", :name "my-text", :value "", :data-x 10, :data-y 20}])
```