docs: clarify Webpack usage when using multiple CSS loaders (#1370)

Author: SimenB

site/docs/integrations/webpack.md

@@ -65,6 +65,8 @@ module.exports = {
 };
 ```
 
+If you already have `css-loader` configured, make sure to add `exclude: /\.vanilla\.css$/i` to that rule's configuration.
+
 ## Configuration
 
 ```js