Update upgrade guide

Author: Flo0807

guides/upgrading/v0.15.md

@@ -54,3 +54,12 @@ Note that it is now also possible to configure the layout as a function:
 use Backpex.LiveResource,
   layout: &MyAppWeb.admin/1
 ```
+
+## Resource.update/6 has been changed
+
+We no longer pass the `fields` to `Resource.update/6`. Therefore, the arity of this functions has changed to `Backpex.Resource.update/5`. This aligns the function `insert` and `update` of the Resource module.
+
+## `:only`/`:except` field option changes
+
+You no longer need to pass `:resource_action` in addition to `:index` to the fields `:only`/`:except` option.
+Before, it was needed to make fields visible behind the backdrop of the resource action modal.