docs: document enhanceApp.js

Author: yyx990803

docs/guide/basic-config.md

@@ -22,3 +22,17 @@ Consult the [Config Reference](../config/) for a full list of options.
 A VuePress theme is responsible for all the layout and interactivity details of your site. VuePress ships with a default theme (you are looking at it right now) which is designed for technical documentation. It exposes a number of options that allow you to customize the navbar, sidebar and homepage, etc. For details, check out the [Default Theme Config](../default-theme-config/) page.
 
 If you wish to develop a custom theme, see [Custom Themes](./custom-themes.md).
+
+## App Level Enhancements
+
+Since the VuePress app is a standard Vue app, you can apply app-level enhancements by creating a file `.vuepress/enhanceApp.js`, which will be imported into the app if it is present. The file should `export default` a hook function which will receive an object containing some app level values. You can use this hook to install additional Vue plugins, register global components, or add additional router hooks:
+
+``` js
+export default ({
+  Vue, // the version of Vue being used in the VuePress app
+  options, // the options for the root Vue instance
+  router // the router instance for the app
+}) => {
+  // ...apply enhancements to the app
+}
+```