feature #5093 Added a new best practices for custom form field types (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Added a new best practices for custom form field types

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #5077

Commits
-------

0696daf Fixed errors, typos and grammar issues. (I managed to fail at everything!)
8eb1240 Added a new best practices for custom form field types

Author: weaverryan

Diff for: best_practices/forms.rst

@@ -207,3 +207,21 @@ Second, we recommend using ``$form->isSubmitted()`` in the ``if`` statement
 for clarity. This isn't technically needed, since ``isValid()`` first calls
 ``isSubmitted()``. But without this, the flow doesn't read well as it *looks*
 like the form is *always* processed (even on the GET request).
+
+Custom Form Field Types
+-----------------------
+
+.. best-practice::
+
+    Add the ``app_`` prefix to your custom form field types to avoid collisions.
+
+Custom form field types inherit from the ``AbstractType`` class, which defines the
+``getName()`` method to configure the name of that form type. These names must
+be unique in the application.
+
+If a custom form type uses the same name as any of the Symfony's built-in form
+types, it will override it. The same happens when the custom form type matches
+any of the types defined by the third-party bundles installed in your application.
+
+Add the ``app_`` prefix to your custom form field types to avoid name collisions
+that can lead to hard to debug errors.