[Doc] add rem guide (#928)

docs/markdown/en-US/quickstart.md

@@ -68,3 +68,9 @@ Vue.use(Vant);
 ```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)

docs/markdown/zh-CN/quickstart.md

@@ -69,3 +69,9 @@ Vue.use(Vant);
 ```shell
 vue init youzan/vue-cli-template-vant projectName
 ```
+
+### rem 适配
+Vant 中的组件样式默认使用`px`作为单位，如果需要使用`rem`作为单位，推荐使用以下两个工具
+
+- [postcss-pxtorem](https://github.com/cuth/postcss-pxtorem) 用于将单位转化为 rem
+- [lib-flexible](https://github.com/amfe/lib-flexible) 用于设置 rem 基准值