Update custom layout documentation for version 1.4.0 (#475)

README.md

@@ -58,13 +58,13 @@ When the user will click on the `Edit this todo` link, as direct response to thi
 ### A note on custom layouts
 
 In order to render turbo frame requests without the application layout, Turbo registers a custom [layout method](https://api.rubyonrails.org/classes/ActionView/Layouts/ClassMethods.html#method-i-layout). 
-If your application uses custom layout resolution, you have to make sure to return `false` for turbo frame requests:
+If your application uses custom layout resolution, you have to make sure to return `""turbo_rails/frame"` (or `false` for TurboRails < 1.4.0) for turbo frame requests:
 
 ```ruby
 layout :custom_layout
 
 def custom_layout
-  return false if turbo_frame_request?
+  return "turbo_rails/frame" if turbo_frame_request?
 
   # ... your custom layout logic
 ```
@@ -81,7 +81,7 @@ you **have** to change it to a layout method in order to conditionally return `f
 layout :custom_layout
 
 def custom_layout
-  return false if turbo_frame_request?
+  return "turbo_rails/frame" if turbo_frame_request?
 
   "some_static_layout"
 ```