Update demo app README with troubleshooting steps (#6003)

Author: prameshj

packages/auth/demo/README.md

@@ -1,6 +1,8 @@
 # Firebase-Auth for web - Auth Demo (Auth Next)
 
-## Prerequisite
+## Prerequisites
+
+You need all the prerequisites mentioned in the [top-level README.](https://github.com/firebase/firebase-js-sdk#prerequisites)
 
 You need to have created a Firebase Project in the
 [Firebase Console](https://firebase.google.com/console/) as well as configured a web app.
@@ -36,14 +38,14 @@ Copy `src/sample-config.js` to `src/config.js`:
 cp src/sample-config.js src/config.js
 ```
 
-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)
+Then copy and paste the Web snippet config found in the console (Project Settings -> Your apps -> SDK setup and Configuration)
 in the `config.js` file.
 
 ## Deploy
 
 Before deploying, you may need to build the auth package:
 ```bash
+yarn
 yarn build:deps
 ```
 
@@ -58,6 +60,20 @@ yarn run demo
 This will compile all the files needed to run Firebase Auth, and start a Firebase server locally at
 [http://localhost:5000](http://localhost:5000).
 
+The demo opens a page like this:
+
+![image](https://user-images.githubusercontent.com/35932340/153662957-41ba6a82-ea15-4084-ad3a-9fd41083efd3.png)
+
+
+This is a developer view of all the supported auth flows. Make sure that the auth flow you are testing is already enabled in your firebase project.
+For example, if you are testing “Sign up with email/password”, your project should allow email/password as a provider. 
+If not, you will see an “auth/operation-not-allowed” error message. 
+
+You can check the enabled providers on the firebase console.
+
+![image](https://user-images.githubusercontent.com/35932340/153662750-c0faf417-07b4-4f0e-93ab-5e0b82e3c793.png)
+
+
 ## Running against Auth Emulator
 
 The demo page by default runs against the actual Auth Backend. To run against the Auth Emulator with mocked endpoints, do the following:
@@ -74,3 +90,68 @@ The demo page by default runs against the actual Auth Backend. To run against th
 yarn run demo:emulator
 ```
 
+## Running against local changes to auth package
+
+By default, the demo runs against the latest release of firebase-auth sdk. This can be modified by:
+
+```
+// packages/auth/demo/package.json
++  "@firebase/auth": "file:..",
+-  "@firebase/auth": "0.18.0",
+```
+
+## Troubleshooting
+
+### Errors about dependency not being installed, example `lerna: command not found`
+  
+  Ensure that you run `yarn` to install dependencies.
+
+### `Failed to get Firebase project <project name>. Please make sure the project exists and your account has permission to access it.`
+
+Logout, re-login and launch the demo
+
+```bash
+firebase logout && firebase login && yarn demo
+```
+
+### `Failed to list firebase projects` when running `firebase use --add`
+
+Logout, re-login and add the project.
+
+```bash
+firebase logout && firebase login && firebase use --add
+```
+
+### `Access to localhost was denied` when accessing the demo app via http://localhost:5000
+
+Most likely this means a different process is binding to localhost:5000.
+You can access the demo app via http://127.0.0.1:5000 or use a different port using `yarn demo --port 5002`
+
+Note - If you use 127.0.0.1 in your browser, you need to allowlist it as a domain for sign in, as shown below.
+
+![image](https://user-images.githubusercontent.com/35932340/153659058-d669055f-b587-4bc2-9f32-323149df50c3.png)
+
+### `hosting: Port 5000 is not open on localhost, could not start Hosting Emulator.`
+
+This can happen when you run `yarn run demo:emulator` if port 5000 is taken.
+Modify auth/demo/firebase.json with a custom port. Pick any port, this example picks 5091.
+
+```
+{
+  //...
+  "emulators": {
+    "hosting": {
+      "host": "",
+      "port": "5091"
+    }
+  }
+}
+```
+
+### Error message about functions
+
+`The Cloud Functions emulator requires the module "firebase-admin" to be installed. This package is in your package.json, but it's not available. You probably need to run "npm install" in your functions directory.
+i  functions: Your functions could not be parsed due to an issue with your node_modules (see above)
+`
+
+Run `npm install` inside the auth/demo/functions directory as mentioned in the error message.