Added a reference to the Bootstrap 3 form theme

Author: javiereguiluz

cookbook/form/form_customization.rst

@@ -93,9 +93,18 @@ rendering a form. In other words, if you want to customize one portion of
 how a form is rendered, you'll import a *theme* which contains a customization
 of the appropriate form fragments.
 
-Symfony comes with a default theme (`form_div_layout.html.twig`_ in Twig and
-``FrameworkBundle:Form`` in PHP) that defines each and every fragment needed
-to render every part of a form.
+Symfony comes with four **built-in form themes** that define each and every
+fragment needed to render every part of a form:
+
+* `form_div_layout.html.twig`_, wraps each form field inside a ``<div>`` element.
+* `form_table_layout.html.twig`_, wraps the entire form inside a ``<table>``
+  element and each form field inside a ``<tr>`` element.
+* `bootstrap_3_layout.html.twig`_, wraps each form field inside a ``<div>`` element
+  with the appropriate CSS classes to apply the default `Bootstrap 3 CSS framework`_
+  styles.
+* `bootstrap_3_horizontal_layout.html.twig`_, it's similar to the previous theme,
+  but the CSS classes applied are the ones used to display the forms horizontally
+  (i.e. the label and the widget in the same row).
 
 In the next section you will learn how to customize a theme by overriding
 some or all of its fragments.
@@ -1059,3 +1068,7 @@ The array passed as the second argument contains form "variables". For
 more details about this concept in Twig, see :ref:`twig-reference-form-variables`.
 
 .. _`form_div_layout.html.twig`: https://github.com/symfony/symfony/blob/master/src/Symfony/Bridge/Twig/Resources/views/Form/form_div_layout.html.twig
+.. _`form_table_layout.html.twig`: https://github.com/symfony/symfony/blob/master/src/Symfony/Bridge/Twig/Resources/views/Form/form_table_layout.html.twig
+.. _`bootstrap_3_layout.html.twig`: https://github.com/symfony/symfony/blob/master/src/Symfony/Bridge/Twig/Resources/views/Form/bootstrap_3_layout.html.twig
+.. _`bootstrap_3_horizontal_layout.html.twig`: https://github.com/symfony/symfony/blob/master/src/Symfony/Bridge/Twig/Resources/views/Form/bootstrap_3_horizontal_layout.html.twig
+.. _`Bootstrap 3 CSS framework`: http://getbootstrap.com/