Add upgrading note regarding the general error structure

Author: tonivdv

UPGRADING.md

@@ -6,8 +6,30 @@ list important BC breaks.
 
 ### upgrading from 0.13.1
 
-* POST routes now pluralize the resource name, ie. /users vs. /user
-* The response for non-valid Forms has changed. See https://github.com/FriendsOfSymfony/FOSRestBundle/blob/master/Resources/doc/2-the-view-layer.md#forms-and-views
+ * POST routes now pluralize the resource name, ie. /users vs. /user
+
+ * The response for non-valid Forms has changed. See https://github.com/FriendsOfSymfony/FOSRestBundle/blob/master/Resources/doc/2-the-view-layer.md#forms-and-views
+
+ * The default error message structure has changed.
+
+    _Old structure_
+
+        {
+          "status": "error",
+          "status_code": 400,
+          "status_text": "Bad Request",
+          "current_content": "",
+          "message": "New comment is not valid."
+        }
+
+    _New structure_
+
+        {
+          "code": 400,
+          "message": "New comment is not valid.";
+        }
+
+    _Alternatively you can inject your own implementation. See https://github.com/FriendsOfSymfony/FOSRestBundle/blob/master/Resources/doc/2-the-view-layer.md#forms-and-views_
 
 ### upgrading from 0.12.0