/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.

Start with /example/README.md and reproduce the setup through npm run build, checking the generated firebase-config.json and the App.jsx:10 error. Update the README to cover creating a web app with firebase apps:create and checking existing apps with firebase apps:list; done means a new reader can complete the example without the reported runtime error.

Written by the indexing model from the issue text.