add firebase auth setup to contributing guide

CONTRIBUTING.md

@@ -35,6 +35,12 @@
    - Click "Generate New Private Key"
    - Save as `serviceAccountKey.json` in the `functions/` directory
 
+1. Enable Firebase Authentication
+
+   - In the Firebase console, go to Authentication
+   - Click on `Email/Password`, enable it and save
+   - Click on `Google`, add a support email and save
+
 ## Building and Running
 
 1. Run `npm install` in the project root directory to install dependencies.