Add a demo for multiple loader (#5122)

English version already merged https://github.com/vuejs/vue-cli/pull/5076/files

Author: FrankFang

docs/zh/guide/webpack.md

@@ -81,6 +81,10 @@ module.exports = {
       .use('graphql-tag/loader')
         .loader('graphql-tag/loader')
         .end()
+      // 你还可以再添加一个 loader
+      .use('other-loader')
+        .loader('other-loader')
+        .end()
   }
 }
 ```