docs: add a warning that css is considered as a side effect (#1485)

closes #1435

Author: haoqunjiang

docs/guide/README.md

@@ -86,3 +86,7 @@ module.exports = {
 ```
 
 Also see [Options Reference](../options.md) for all available loader options.
+
+::: warning
+If you are developing a library or in a monorepo, please be aware that CSS imports **are side effects**. Make sure to **remove** `"sideEffects": false` in the `package.json`, otherwise CSS chunks will be dropped by webpack in production builds.
+:::