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

[iku000888]

There is the following undocumented behaviour that is works well with bootstrap, which would be nice to have it right in the readme.

(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}])