Symfony表单单选按钮其他

I need radio button with field other and when user select other and write some text in field and submit form I set this is text in field. I have form:

 public function buildForm(FormBuilderInterface $builder, array $options)
{
       $builder
            ->add('reason', 'choice', array(
                'choices' => array(
                    'I’m not interested in projects, I receive here' => 'I’m not interested in projects, I receive here',
                    'I really don’t like your service' => 'I really don’t like your service',
                    'I never signed up for this mailing list' => 'I never signed up for this mailing list',
                    'I no longer want to receive these emails' => 'I no longer want to receive these emails',
                    'other' => 'other'
                ),
                'multiple' => false,
                'expanded' => true,
                'required' => true
            ))

        ->add('submit', 'submit');
    ;
}

in twig

{{ form_start(form, {'action': path('developers_resubscribe', {'token': token, 'id': unsubscribe.id }), 'method': 'POST'}) }}
{{ form_errors(form) }}

{{ form_row(form.reason) }}

{{ form_row(form.submit, { 'label': 'Submit me' }) }}
{{ form_end(form) }}

and I not understand how its add in this form field when user select other for user reason - like this screen

And my action:

        if ($request->isMethod('POST')) {

        $form->bind($request);
        if ($form->isValid()) {

            $em->persist($unsubscribe);
            $em->flush();

            return $this->redirect($this->generateUrl('developers_resubscribe',  array('token' => $token, 'id' => $unsubscribe->getId())));
        }
    }

Help please