adding note to help in upgrade path

Author: weaverryan

README.md

@@ -39,6 +39,20 @@ import { startStimulusApp } from '@symfony/stimulus-bridge';
 export const app = startStimulusApp(require.context('./controllers', true, /\.(j|t)sx?$/));
 ```
 
+If you get this error:
+
+> ./assets/bootstrap.js contains a reference to the file @symfony/autoimport.
+> This file can not be found.
+
+Remove the following line in the mentioned file: it's not needed anymore:
+
+```diff
+// assets/bootstrap.js
+
+// ...
+- import '@symfony/autoimport';
+```
+
 ## Run tests
 
 ```sh