Add instructions for how to add external package documentation to Plone 6 Documentation

Author: stevepiercy

Makefile

@@ -39,7 +39,6 @@ bin/python:
 	bin/pip install -r requirements.txt
 
 docs/volto:
-	# git submodule add [email protected]:plone/volto.git submodules/volto
 	git submodule init; \
 	git submodule update; \
 	ln -s ../submodules/volto/docs/source ./docs/volto

docs/contributing/authors.md

@@ -15,6 +15,28 @@ It covers configuring quality checks and syntax for writing markup that is of pa
 For general markup syntax, see {doc}`writing-docs-guide`.
 
 
+(authors-import-docs-submodule-label)=
+
+## Importing external docs with submodules
+
+To add an external package to Plone Documentation, we use git submodules.
+We did this with Volto documentation.
+Your package must be available under the Plone GitHub organization.
+
+Inside the repository `plone/documentation`, add a git submodule that points to your project.
+
+```shell
+git submodule add [email protected]:plone/my_package.git submodules/my_package
+```
+
+To make it easier for other contributors to work with your project, update the following files, using `volto` as a model.
+ 
+-   `Makefile` targets  `docs/my_package` and `deps`
+-   The documentation section {ref}`contributing-documentation-and-volto-label`
+
+Commit and push your changes to a remote, and submit a pull request against [`plone/documentation@6-dev`](https://github.com/plone/documentation/compare/6-dev...my_package).
+
+
 (authors-html-meta-data-label)=
 
 ## HTML and Open Graph Metadata