Update demo app README with troubleshooting steps

[prameshj]

Updated the readme with screenshots and troubleshooting steps.

@sam-gc @renkelvin

[changeset-bot]

⚠️ No Changeset found

Latest commit: 8e4dfa9

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (fde5adf) and merge commit (c98aa30).

Test Logs

• Base (fde5adf): https://github.com/firebase/firebase-js-sdk/actions/runs/3527399174
• Merge (c98aa30): https://github.com/firebase/firebase-js-sdk/actions/runs/3568491332

1. https://storage.googleapis.com/firebase-sdk-metric-reports/kHWneCPU0B.html

[google-oss-bot]

Size Analysis Report ¹

Affected Products

No changes between base commit (fde5adf) and merge commit (c98aa30).

Test Logs

• Base (fde5adf): https://github.com/firebase/firebase-js-sdk/actions/runs/3527399174
• Merge (c98aa30): https://github.com/firebase/firebase-js-sdk/actions/runs/3568491332

1. https://storage.googleapis.com/firebase-sdk-metric-reports/hdyrBZaFG2.html

[Xiaoshouzi-gh]

Content in the copy config section needs more clarification.
From Then copy and paste the Web snippet config found in the console (either by clicking "Add Firebase to your web app" button in your Project overview, or clicking the "Web setup" button in the Auth page) in the `config.js` file.

to

Redirect to Firebase console, click on Project Overview from the side bar.  Click on Add app then choose `Web`.  Fill in App nickname and leave the `Also set up Fiebase Hosting for this app` unchecked for now.  Click on Register app.  In the Add Firebase SDK section, copy over the firebaseconfig object to your `config.js` file config object.

my guess is - before register app, it will show 0 app in the project. We might not able to find this config anywhere.

[prameshj]

Content in the copy config section needs more clarification. From Then copy and paste the Web snippet config found in the console (either by clicking "Add Firebase to your web app" button in your Project overview, or clicking the "Web setup" button in the Auth page) in the `config.js` file.

to

Redirect to Firebase console, click on Project Overview from the side bar.  Click on Add app then choose `Web`.  Fill in App nickname and leave the `Also set up Fiebase Hosting for this app` unchecked for now.  Click on Register app.  In the Add Firebase SDK section, copy over the firebaseconfig object to your `config.js` file config object.

my guess is - before register app, it will show 0 app in the project. We might not able to find this config anywhere.

This doc specifies You need to have created a Firebase Project in the [Firebase Console](https://firebase.google.com/console/) as well as configured a web app. as a pre-req, so the user should have one app here. Is is ok to skip this?