Update database config explanation in with-firebase-app example (#20709)

I made the following changes.

- Addition of procedure for setting private_key
- Addition of procedure for creating a database and setting database_url

examples/with-firebase-authentication/README.md

@@ -18,8 +18,9 @@ Set up Firebase:
 
 - Create a project at the [Firebase console](https://console.firebase.google.com/).
 - Copy the contents of `.env.local.example` into a new file called `.env.local`
-- Get your account credentials from the Firebase console at _Project settings > Service accounts_, where you can click on _Generate new private key_ and download the credentials as a json file. It will contain keys such as `project_id`, `client_email` and `client_id`. Set them as environment variables in the `.env.local` file at the root of this project.
-- Get your authentication credentials from the Firebase console under _Project settings > General> Your apps_ Add a new web app if you don't already have one. Under _Firebase SDK snippet_ choose _Config_ to get the configuration as JSON. It will include keys like `apiKey`, `authDomain` and `databaseUrl`. Set the appropriate environment variables in the `.env.local` file at the root of this project.
+- Get your account credentials from the Firebase console at _Project settings > Service accounts_, where you can click on _Generate new private key_ and download the credentials as a json file. It will contain keys such as `project_id`, `client_email` and `private_key`. Set them as environment variables in the `.env.local` file at the root of this project.
+- Get your authentication credentials from the Firebase console under _Project settings > General> Your apps_ Add a new web app if you don't already have one. Under _Firebase SDK snippet_ choose _Config_ to get the configuration as JSON. It will include keys like `apiKey` and `authDomain`. Set the appropriate environment variables in the `.env.local` file at the root of this project.
+- Go to **Develop**, click on **Realtime Database** and create a database if you don't already have one. Under _data_ get `databaseUrl`(e.g. `https://[dbname].firebaseio.com/`). Set the appropriate environment variables in the `.env.local` file at the root of this project.
 - Go to **Develop**, click on **Authentication** and in the **Sign-in method** tab enable authentication for the app.
 
 Install it and run: