Add step to set up support email (#330)

To address feedback in #324

auth/README.md

@@ -25,6 +25,7 @@ Getting Started
 ---------------
 
  1. Create a Firebase project on the [Firebase Console](https://console.firebase.google.com).
+ 1. Add a support email to your project in the [settings page](https://console.firebase.google.com/u/0/project/_/settings/general/). Some auth methods won't work without this.
  1. Enable the authentication method you want to use by going to the **Authentication** section in the **SIGN-IN METHOD** tab - you don't need to enable custom auth.
      - For **Custom Auth**, generate a Service Account credentials in your [Firebase Console > Project Settings > Service Accounts](https://console.firebase.google.com/project/_/settings/serviceaccounts/adminsdk), and click on **GENERATE NEW PRIVATE KEYS**. You will need it in the [example token generator](exampletokengenerator/auth.html).
      - For **Facebook**, **Twitter** and **GitHub** you will need to create an application as a developer on their respective developer platform, whitelist `https://<project_id>.firebaseapp.com/__/auth/handler` for auth redirects and enable and setup the app's credentials in the **Firebase Console > Authentication > SIGN-IN METHOD**.