add gtag

Author: mrholek

public/index.html

@@ -23,6 +23,17 @@
       work correctly both with client-side routing and a non-root public URL.
       Learn how to configure a non-root public URL by running `npm run build`.
     -->
+    <!-- Global site tag (gtag.js) - Google Analytics -->
+    <script async src="https://www.googletagmanager.com/gtag/js?id=UA-118965717-3"></script>
+    <script>
+      window.dataLayer = window.dataLayer || [];
+      function gtag(){dataLayer.push(arguments);}
+      gtag('js', new Date());
+      // Shared ID
+      gtag('config', 'UA-118965717-3');
+      // React.js ID
+      gtag('config', 'UA-118965717-6');
+    </script>
   </head>
 
   <!-- BODY options, add following classes to body to change options