Update README.md

Add note for #4292

README.md

@@ -11,6 +11,8 @@ frequently (but not continuously).
 This is currently the recommended way to consume the documentation. The tooling
 to build and run this site is [available on github](https://github.com/Raku/doc-website).
 
+This repository is not intended to be installed as a module.
+
 ## README in other languages
 
   [日本語](resources/i18n/jp/README.jp.md)