docs: add notes about keep_files

Note that users who are using a Static Site Generator do not need this option in most cases. Please reconsider your project structure and building scripts, or use a built-in feature of a Static Site Generator before you enable this flag.

- [Static Files | Hugo](https://gohugo.io/content-management/static-files/)
- [Using the Static Folder | Gatsby](https://www.gatsbyjs.com/docs/static-folder/)

Related issues:

- #507
- #509

Author: peaceiris

README.md

@@ -332,6 +332,11 @@ For example:
 
 By default, existing files in the publish branch are removed before adding the ones from publish dir. If you want the action to add new files but leave existing ones untouched, set the optional parameter `keep_files` to `true`.
 
+Note that users who are using a Static Site Generator do not need this option in most cases. Please reconsider your project structure and building scripts, or use a built-in feature of a Static Site Generator before you enable this flag.
+
+- [Static Files | Hugo](https://gohugo.io/content-management/static-files/)
+- [Using the Static Folder | Gatsby](https://www.gatsbyjs.com/docs/static-folder/)
+
 For example:
 
 ```yaml