/example/README.md doesn't mention creating an app first

I followed example README.md exactly, and result was the example didn't work, with a runtime JS error:

```
Uncaught TypeError: Cannot read properties of undefined (reading 'sdkConfig')
at Object. (App.jsx:10:58)
```

Cause of the above is `npm run build` produced `firebase-config.json` that contained an error (with misleading message, firebase/firebase-tools/issues/4007) because I hadn't created a "web" app yet in the Firebase project.

The readme, since it has readers create a new Firebase project, should include a `firebase apps:create` invocation, to ensure there's a web app available. Could also mention using `firebase apps:list`, in case the Firebase project is not new, already has 2 apps, which I presume causes the same error(?) - I can imagine people trying to use most of these steps with an existing project.

Comienza con /example/README.md y reproduce la configuración hasta npm run build, comprobando el firebase-config.json generado y el error de App.jsx:10. Actualiza la README para incluir la creación de una aplicación web con firebase apps:create y la comprobación de las aplicaciones existentes con firebase apps:list; se considera terminado cuando un lector nuevo puede completar el ejemplo sin el error de ejecución informado.

Escrito por el modelo de indexación a partir del texto del issue.