docs: update CDN guide, resolve #161

Author: antfu

README.md

@@ -9,29 +9,44 @@ English | [中文](./README.zh-CN.md) · [**Composition API Docs**](https://comp
 
 ## Installation
 
+### NPM
+
 ```bash
 npm install @vue/composition-api
 # or
 yarn add @vue/composition-api
 ```
 
-**CDN**
+You must install `@vue/composition-api` via `Vue.use()` before you can use the [Composition API](https://composition-api.vuejs.org/) to compose your component.
+
+```js
+import Vue from 'vue'
+import VueCompositionApi from '@vue/composition-api'
+
+Vue.use(VueCompositionApi)
+```
+
+```js
+// in components
+import { ref, reactive } from '@vue/composition-api'
+```
+
+### CDN
 
 ```html
 <script src="https://unpkg.com/@vue/composition-api/dist/vue-composition-api.umd.js"></script>
 ```
 
 The package will be exposed to global variable `window.vueCompositionApi`
 
-## Usage
-
-You must install `@vue/composition-api` via `Vue.use()` before you can use the [Composition API](https://composition-api.vuejs.org/) to compose your component.
-
 ```js
-import Vue from 'vue'
-import VueCompositionApi from '@vue/composition-api'
+// install the plugin
+Vue.use(vueCompositionApi.default)
+```
 
-Vue.use(VueCompositionApi)
+```js
+// use the APIs
+const { ref, reactive } = vueCompositionApi
 ```