explain how to override form types

Author: Bart van den Burg

cookbook/bundles/override.rst

@@ -45,7 +45,19 @@ In progress...
 Forms
 -----
 
-In progress...
+In order to override a form type, it has to be registered as a service (meaning
+it is tagged as "form.type"). You can then override it as you would override any
+service as explained in "Services & Configuration". This, of course, will only
+work if the type is referred to by its alias rather than being instantiated,
+e.g.:
+
+.. code-block:: php
+    $builder->add('name', 'custom_type');
+
+rather than
+
+.. code-block:: php
+    $builder->add('name', new CustomType());
 
 Validation metadata
 -------------------