feat(pwa): add sw.js

docs/index.html

@@ -32,6 +32,10 @@
       '/changelog': 'https://raw.githubusercontent.com/QingWei-Li/docsify/master/CHANGELOG'
     }
   }
+
+  if (typeof navigator.serviceWorker !== 'undefined') {
+    navigator.serviceWorker.register('sw.js')
+  }
 </script>
 <script src="//unpkg.com/docsify/lib/docsify.min.js"></script>
 <script src="//unpkg.com/docsify/lib/plugins/search.min.js"></script>