Better markdown in README

[ci skip]
Add `form_for` helper
Add `text_field` form helper
Add :tag option to `panel` helper
Add `fieldset` form helper
Add `fields_for` form helper
Add all other form helpers

For Rails 3 and 4: email file number password phone search telephone text url

For Rails 4 only: color date datetime datetime_local month time week
Add `submit` form helper
Add `legend` form helper
Add `check_box` form helper
Add `inline-label` option to `check_box`
Add `radio_button` form helper
Add `select` form helper
Add `static_control` form helper
Add errors to `text_field` form helper

Rather than continuing with the "3 builders" approach, we are
better off having just one builder and adding all the cases
about layout (basic, horizontal, inline) and error display.

To display inline error messages:

    form_for :user, errors: {inline: true} # default

To display error icons in the text field:

    form_for :user, errors: {icons: true} # default

The next commits will do the same for the remaining field types.
Extract common code in BaseHelper
Add errors to every "textual" field helper
Add errors to `check_box` form helper