Add note about doc-related pull requests in contribution guide

CONTRIBUTING.md

@@ -7,6 +7,8 @@ helping to manage issues, etc.
 
 The Hugo community and maintainers are [very active](https://github.com/gohugoio/hugo/pulse/monthly) and helpful, and the project benefits greatly from this activity. We created a [step by step guide](https://gohugo.io/tutorials/how-to-contribute-to-hugo/) if you're unfamiliar with GitHub or contributing to open source projects in general.
 
+*Note that this repository only contains the actual source code of Hugo. For documentation-related pull requests / issues please refer to the [hugoDocs](https://github.com/gohugoio/hugodocs) repository.*
+
 ## Table of Contents
 
 * [Asking Support Questions](#asking-support-questions)