added service key instructions

Author: ixetanet

README.md

@@ -18,6 +18,18 @@ Add your Firebase configuration to appsettings.json:
 }
 ```
 
+## Download Service Account Key
+
+Generate and configure your Firebase service account private key:
+
+1. Navigate to your Firebase project in the Firebase console
+2. Open **Project settings**
+3. Select the **Service accounts** tab
+4. Click **Generate new private key** in the Firebase Admin SDK section
+5. Confirm to download the JSON key file
+6. Add the JSON file to your project (e.g., in a `keys` folder)
+7. Configure the file to copy to output directory in your project settings
+
 ## Configure the Firebase Console
 
 Configure the Firebase email template action URL to redirect to your Xams application: