Adds note on subfolders to history mode samples

fixes vuejs#2863

Author: AVGP

docs/guide/essentials/history-mode.md

@@ -19,6 +19,8 @@ Not to worry: To fix the issue, all you need to do is add a simple catch-all fal
 
 ## Example Server Configurations
 
+**Note**: The following examples assume you are serving your app from the root folder. If you deploy to a subfolder, see [the `publicPath` option of Vue CLI](https://cli.vuejs.org/config/#publicpath) and the related [`base` property of the router](https://router.vuejs.org/api/#base).
+
 #### Apache
 
 ```apache