Update README: how to contribute to Volto, plone.api, plone.restapi documentation

Author: ksuess

README.md

@@ -31,6 +31,12 @@ The community is writing guides, installation instructions, and everything you n
 - [Issue Tracker](https://github.com/plone/documentation/issues)
 - [Source Code](https://github.com/plone/documentation/tree/6-dev)
 
+```{admonition} Contributing to Volto documentation, plone.api and plone.restapi documentation
+The main part of the Plone documentation resides in this repository.
+So you are contributing here. But we also include documentation of some Plone core packages: Volto, plone.api and plone.restapi.
+If you want to contribute to them, you are welcome to do so, then please open pull requests in their repository.
+Here is the explanation on how to contribute to the documentation of the mentioned packages: [Editing external package documentation](https://6.dev-docs.plone.org/contributing/index.html#contributing-editing-external-package-documentation-label)
+```
 
 ## Support