add google tag

Author: henry-yeh

index.html

@@ -39,6 +39,15 @@
   <script src="static/js/bulma-slider.min.js"></script>
   <script src="static/js/index.js"></script>
 
+  <!-- Google tag (gtag.js) -->
+  <script async src="https://www.googletagmanager.com/gtag/js?id=G-HWFCQDEG04"></script>
+  <script>
+    window.dataLayer = window.dataLayer || [];
+    function gtag(){dataLayer.push(arguments);}
+    gtag('js', new Date());
+    gtag('config', 'G-HWFCQDEG04');
+  </script>
+
 <style>
     .publication-title {
       font-size: 1.4rem;