Update upgrade guide

guides/upgrading/v0.11.md

@@ -17,3 +17,31 @@ Update Backpex to the latest version:
 In case you are using `Backpex.Resource` or one of the `Backpex.Adapter` modules (`Backpex.Adapters.Ecto` or
 `Backpex.Adapters.Ash`) directly check out the updated function definitions. This will also apply in case you built your
 own adapter.
+
+## Make sure to cover all cases with the `item_query/3` function
+
+We have removed code that ensures that a fallback item query function is always added to your LiveResource. 
+
+Make sure to always cover all possible cases or add a fallback `item_query/3` function that just returns the query.
+
+For example:
+
+```elixir
+# in your resource configuration file (live resource)
+use Backpex.LiveResource,
+  # ...other options
+  adapter_config: [
+    # ...other adapter options
+    item_query: &__MODULE__.item_query/3
+  ]
+
+  def item_query(query, :index, _assigns) do
+    query
+    |> where([post], post.published)
+  end
+
+  # make sure to add this fallback function
+  def item_query(query, _live_action, _assigns) do
+    query
+  end
+```