/**
 * Implements hook_field_attach_form().
 */
function {{ machine_name }}_field_attach_form($entity_type, $entity, &$form, &$form_state, $langcode) {
  // Add a checkbox allowing a given field to be emptied.
  // See hook_field_attach_submit() for the corresponding processing code.
  $form['empty_field_foo'] = array(
    '#type' => 'checkbox',
    '#title' => t("Empty the 'field_foo' field"),
  );
}