improve docs

Author: binaryify

docs/.vuepress/config.js

@@ -1,3 +1,11 @@
 module.exports = {
-  base: '/vue-custom-scrollbar/'
+  base: '/vue-custom-scrollbar/',
+  themeConfig: {
+    nav: [
+      {
+        text: 'GitHub',
+        link: 'https://github.com/Binaryify/vue-custom-scrollbar'
+      }
+    ]
+  }
 }