mention navigateFallback in the pwa docs (#4973)

docs/core-plugins/pwa.md

@@ -31,6 +31,11 @@ file, or the `"vue"` field in `package.json`.
 
   These options are passed on through to the underlying `workbox-webpack-plugin`.
 
+  If you're using the App Shell pattern with the `GenerateSW` mode, you can configure your entry point like this to make sure all your pages load offline:
+  ```js
+  navigateFallback: 'index.html'
+  ```
+
   For more information on what values are supported, please see the guide for
   [`GenerateSW`](https://developers.google.com/web/tools/workbox/modules/workbox-webpack-plugin#full_generatesw_config)
   or for [`InjectManifest`](https://developers.google.com/web/tools/workbox/modules/workbox-webpack-plugin#full_injectmanifest_config).