Add note on 2nd rtdb reference.

Author: jonathanedey

CONTRIBUTING.md

@@ -168,6 +168,11 @@ Set up your Firebase project as follows:
    1. Go to the Firebase Console, and select **Realtime Database** from the **Build** menu.
    2. Click on the **Create Database** button. You can choose to set up the Realtime Database
    either in the locked mode or in the test mode.
+
+   > **Note:** Integration tests are not ran against the default Realtime Database reference and are
+   instead ran against a database created at `https://{PROJECT_ID}.firebaseio.com`.
+   This second Realtime Database referance is created in the following steps.
+
    3. In the **Data** tab click on the kebab menu (3 dots) and select **Create Database**.
    4. Enter your Project ID (Found in the **General** tab in **Account Settings**) as the
    **Realtime Database reference**. Again, you can choose to set up the Realtime Database