note form requirements in autocomplete

Author: MrYamous

src/Autocomplete/doc/index.rst

@@ -34,7 +34,13 @@ Usage in a Form (without Ajax)
 ------------------------------
 
 Any ``ChoiceType`` or ``EntityType`` can be transformed into a
-Tom Select-powered UI control by adding the ``autocomplete`` option:
+Tom Select-powered UI control by adding the ``autocomplete`` option.
+
+In a new project you may need to install `Symfony Form`_:
+
+.. code-block:: terminal
+
+    $ composer require symfony/form
 
 .. code-block:: diff
 
@@ -747,3 +753,4 @@ the Symfony framework: https://symfony.com/doc/current/contributing/code/bc.html
 .. _`controller.ts`: https://github.com/symfony/ux/blob/2.x/src/Autocomplete/assets/src/controller.ts
 .. _`Tom Select Render Templates`: https://tom-select.js.org/docs/#render-templates
 .. _`Tom Select Option Group`: https://tom-select.js.org/examples/optgroups/
+.. _`Symfony form`: https://symfony.com/doc/current/forms.html