add analytics

Author: rainliu

index.html

@@ -163,5 +163,13 @@
       <img src="./res/animated_ferris_white.gif">
   </div>
 </body>
+<!-- Global site tag (gtag.js) - Google Analytics -->
+<script async src="https://www.googletagmanager.com/gtag/js?id=G-BHTZSJEX72"></script>
+<script>
+  window.dataLayer = window.dataLayer || [];
+  function gtag(){dataLayer.push(arguments);}
+  gtag('js', new Date());
 
+  gtag('config', 'G-BHTZSJEX72');
+</script>
 </html>