Add upgrade guide

Author: wouterj

UPGRADE-2.1.md

@@ -0,0 +1,24 @@
+# Upgrade from 2.0 to 2.1
+
+## Model
+
+* Removed `setParent()`/`getParent()` from the model `MenuNode`. Use
+  `setParentObject()`/`getParentObject()` instead.
+
+  **Before**
+  ```php
+  use Symfony\Cmf\Bundle\MenuBundle\Model\MenuNode;
+  // ...
+
+  $node = new MenuNode('some name');
+  $node->setParent($menuRoot);
+  ```
+
+  **After**
+  ```php
+  use Symfony\Cmf\Bundle\MenuBundle\Model\MenuNode;
+  // ...
+
+  $node = new MenuNode('some name');
+  $node->setParentObject($menuRoot);
+  ```