Show example on using a DateTimeImmutable object

Add an example on using DateTimeImmutable through the `input` option.

Author: pavlakis

reference/forms/types/date.rst

@@ -65,6 +65,14 @@ field as **three different choice fields**::
 If your underlying date is *not* a ``DateTime`` object (e.g. it's a unix timestamp),
 configure the `input`_ option.
 
+When working with a ``DateTimeImmutable`` object, add this as an ``input`` option::
+
+    $builder->add('publishedAt', DateType::class, [
+        'widget' => 'choice',
+        'input'  => 'datetime_immutable'
+    ]);
+
+
 Rendering a single HTML5 Textbox
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~