Update README.md (#834)

Include instructions on adding firebase-auth SDK CDN along with the initialization snippet.

Author: Ready4theCrush

README.md

@@ -195,7 +195,9 @@ for a more in-depth example, showcasing a Single Page Application mode.
     <title>Sample FirebaseUI App</title>
     <!-- *******************************************************************************************
        * TODO(DEVELOPER): Paste the initialization snippet from:
-       * Firebase Console > Overview > Add Firebase to your web app. *
+       * Firebase Console > Overview > Add Firebase to your web app.
+       * In addition, include the firebase-auth SDK:
+       * <script src="https://www.gstatic.com/firebasejs/[FIREBASE VERSION USED IN SNIPPET]/firebase-auth.js"></script> *
        ***************************************************************************************** -->
     <script src="https://www.gstatic.com/firebasejs/ui/4.8.0/firebase-ui-auth.js"></script>
     <link type="text/css" rel="stylesheet" href="https://www.gstatic.com/firebasejs/ui/4.8.0/firebase-ui-auth.css" />