add upgrade note

UPGRADE-3.x.md

@@ -1,6 +1,14 @@
 UPGRADE 3.x
 ===========
 
+UPGRADE FROM 3.12 to 3.x
+========================
+
+### SonataEasyExtends is deprecated
+
+Registering `SonataEasyExtendsBundle` bundle is now deprecated, you should not register it,
+and instead register the `SonataDoctrineBundle` bundle.
+
 UPGRADE FROM 3.1 to 3.2
 =======================
 
@@ -12,6 +20,6 @@ and should be done by overriding `getFormBuilder` instead.
 
 ### Tests
 
-All files under the ``Tests`` directory are now correctly handled as internal test classes. 
-You can't extend them anymore, because they are only loaded when running internal tests. 
+All files under the ``Tests`` directory are now correctly handled as internal test classes.
+You can't extend them anymore, because they are only loaded when running internal tests.
 More information can be found in the [composer docs](https://getcomposer.org/doc/04-schema.md#autoload-dev).