Add customizing solutions to export action docs (#4294)

Resources/doc/reference/action_export.rst

@@ -19,13 +19,80 @@ All field names are translated by default.
 An internal mechanism checks if a field matching the translator strategy label exists in the current translation file
 and will use the field name as a fallback.
 
+Customizing export
+~~~~~~~~~~~~~~~~~~
+
+To customize available export formats just overwrite AbstractAdmin::getExportFormats()
+
+.. code-block:: php
+
+    <?php
+    // src/AppBundle/Admin/PersonAdmin.php
+
+    class PersonAdmin extends AbstractAdmin
+    {
+        /**
+         * {@inheritdoc}
+         */
+        public function getExportFormats()
+        {
+            return array(
+                'json', 'xml', 'csv', 'xls',
+            );
+        }
+    }
+
+If you want to customize the list of fields to export, overwrite AbstractAdmin::getExportFields() like
+this:
+
+.. code-block:: php
+
+    <?php
+    // src/AppBundle/Admin/PersonAdmin.php
+
+    class PersonAdmin extends AbstractAdmin
+    {
+        /**
+         * @return array
+         */
+        public function getExportFields() {
+            return array(
+                'Id' => 'id',
+                'First Name' => 'firstName',
+                'Last Name' => 'lastName',
+                'Contact' => 'contact.phone',
+            );
+        }
+    }
+
+.. note::
+
+    Note that you can use `contact.phone` to access the `phone` property of `Contact` entity
+
+To add more customization to your export you can overwrite AbstractAdmin::getDataSourceIterator().
+Supposing you want to change date format in your export file. You can do it like this:
+
+.. code-block:: php
+
+    <?php
+    // src/AppBundle/Admin/PersonAdmin.php
+
+    class PersonAdmin extends AbstractAdmin
+    {
+        public function getDataSourceIterator()
+        {
+            $iterator = parent::getDataSourceIterator();
+            $iterator->setDateTimeFormat('d/m/Y'); //change this to suit your needs
+            return $iterator;
+        }
+    }
+
 .. note::
 
     **TODO**:
     * any global (yml) options that affect the export actions
-    * how to disable (some of) the default formats
     * how to add new export formats
-    * customising the templates used to render the output
     * customising the query used to fetch the results
 
 .. _`issues on Github`: https://github.com/sonata-project/SonataAdminBundle/issues/1519
+.. _`the exporter bundle documentation`: https://github.com/sonata-project/exporter/blob/1.x/docs/reference/symfony.rst