WC: Add attributes to checkout fields

gistfile1.txt

// Add attributes to checkout fields
function wc_add_checkout_add_ons_attributes( $checkout_fields ) {

    $add_on_id = 2; // change number to correct add-on id

    if ( isset( $checkout_fields['add_ons'][ 'wc_checkout_add_ons_' . $add_on_id ] ) ) {
        $checkout_fields['add_ons'][ 'wc_checkout_add_ons_' . $add_on_id ]['placeholder']   = "Add a short message for a handwritten card (optional)";
        //Adds placeholder text
    }

    return $checkout_fields;
}
add_filter( 'woocommerce_checkout_fields', 'wc_add_checkout_add_ons_attributes', 20 );

/** http://docs.woothemes.com/document/woocommerce-checkout-add-ons-developer-docs/ **/