[Doc] improve quickstart (#1187)

docs/markdown/quickstart.en-US.md

@@ -1,14 +1,42 @@
 ## Quickstart
 
+### Starter kit
+
+Use [vue-cli](https://github.com/vuejs/vue-cli) to create a vant project.
+
+```shell
+vue init youzan/vue-cli-template-vant projectName
+```
+
 ### Install
 
+#### NPM
+
 ```shell
 npm i vant -S
 ```
 
+#### YARN
+
+```shell
+yarn add vant
+```
+
+#### CDN
+
+```html
+<!-- import style -->
+<link rel="stylesheet" href="https://unpkg.com/vant/lib/vant-css/index.css" />
+
+<!-- import script -->
+<script src="https://unpkg.com/vue/dist/vue.min.js"></script>
+<script src="https://unpkg.com/vant/lib/vant.min.js"></script>
+```
+
 ### Usage
 
 #### 1. Use [babel-plugin-import](https://github.com/ant-design/babel-plugin-import) (Recommended)
+
 ```bash
 # Install babel-plugin-import
 npm i babel-plugin-import -D
@@ -43,7 +71,6 @@ import 'vant/lib/vant-css/button.css';
 ```
  
 #### 3. Import all components
-If you configured babel-plugin-import, you won't be allowed to import all components.
 
 ```js
 import Vue from 'vue';
@@ -53,24 +80,28 @@ import 'vant/lib/vant-css/index.css';
 Vue.use(Vant);
 ```
 
-### CDN
+> If you configured babel-plugin-import, you won't be allowed to import all components.
 
-```html
-<!-- import style -->
-<link rel="stylesheet" href="https://unpkg.com/vant/lib/vant-css/index.css" />
 
-<!-- import script -->
-<script src="https://unpkg.com/vant/lib/vant.min.js"></script>
-```
+### Rem units
 
-### vue-cli template
-
-```shell
-vue init youzan/vue-cli-template-vant projectName
-```
-
-### rem units
 Vant use `px` as size units by default，you can use tools such as `postcss-pxtorem` to transform units to `rem`.
 
 - [postcss-pxtorem](https://github.com/cuth/postcss-pxtorem)
 - [lib-flexible](https://github.com/amfe/lib-flexible)
+
+postcss config example:
+
+```js
+module.exports = {
+  plugins: {
+    'autoprefixer': {
+      browsers: ['Android >= 4.0', 'iOS >= 7']
+    }
+    'postcss-pxtorem': {
+      rootValue: 37.5,
+      propList: ['*']
+    }
+  }
+}
+```