docs: warn of styling config restriction

Author: billyyyyy3320

packages/docs/docs/config/README.md

@@ -187,6 +187,13 @@ VuePress provides a convenient way to add extra styles. You can create a `.vuepr
 
 - [Why can’t `palette.styl` and `index.styl` merge into one API?](../faq/README.md#why-can-t-palette-styl-and-index-styl-merge-into-one-api)
 
+::: warning
+During the process, both `palette.styl` and `index.styl` will be imported by temp files in temporary directory for client. Therefore, the the normal `.css` style sheets cannot be resolve with relative path.
+
+However, you’re able to import/require other Stylus style sheets. They’re not under the restriction.
+
+:::
+
 ## Theming
 
 ### theme
@@ -321,7 +328,7 @@ This option is also included in [Plugin API](../plugin/option-api.md#extendmarkd
 - Default: `['h2', 'h3']`
 
 While preparing the page, headers are extracted from the Markdown file and stored in `this.$page.headers`. By default, VuePress will extract `h2` and `h3` elements for you. You can override the headers it pulls out in your `markdown` options.
- 
+
 ``` js
 module.exports = {
   markdown: {