Update migration guide

Addresses remix-run#4623

docs/guides/migrating-react-router-app.md

@@ -184,6 +184,8 @@ Inside of your `$.jsx` file, all we need to do is export the code from our old r
 export { default } from "~/old-app/app";
 ```
 
+<docs-warning><strong>Important:</strong> make sure that you still have the `app/routes/index.[jsx|tsx]` file. Remix will not automatically render your `/` React Router fallback route, which means a blank page will be rendered without the index file in the `routes` directory.</docs-warning>
+
 ## Replacing the bundler with Remix
 
 Remix provides its own bundler and CLI tools for development and building your app. Chances are your app used something like Create React App to bootstrap, or perhaps you have a custom build set up with Webpack.