Added data transformers types image

Author: lmammino

cookbook/form/data_transformers.rst

@@ -184,6 +184,9 @@ In the above example, the transformer was used as a "model" transformer.
 In fact, there are two different type of transformers and three different
 types of underlying data.
 
+.. image:: /images/cookbook/form/DataTransformersTypes.png
+   :align: center
+
 In any form, the 3 different types of data are:
 
 1) **Model data** - This is the data in the format used in your application