[Doc] Theme: babel version tip (#2106)

docs/markdown/theme.en-US.md

@@ -40,6 +40,7 @@ First you should import the less source file to your project. you can use babel-
 
 ```js
 // configure babel plugin in babel.config.js
+// For users who use babel6, please manually import less file
 module.exports = {
   plugins: [
     [

docs/markdown/theme.zh-CN.md

@@ -45,6 +45,7 @@ Vant 支持通过 babel 插件按需引入和手动引入两种方式，推荐
 
 ```js
 // 在 babel.config.js 中配置按需引入样式源文件
+// 注意：babel6 不支持按需引入样式，请手动引入
 module.exports = {
   plugins: [
     [