docs: add advanced usage guide

docs/markdown/quickstart.en-US.md

@@ -122,30 +122,3 @@ Vue.use(Vant);
 ```
 
 > If you configured babel-plugin-import, you won't be allowed to import all components.
-
-## Other
-
-### 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
-
-postcss config example:
-
-```js
-module.exports = {
-  plugins: {
-    autoprefixer: {
-      browsers: ['Android >= 4.0', 'iOS >= 8'],
-    },
-    'postcss-pxtorem': {
-      rootValue: 37.5,
-      propList: ['*'],
-    },
-  },
-};
-```