include markdown-include mkdocs dependency

docs/solo/documenting.md

@@ -3,7 +3,7 @@ Documentation of the `master` branch is deployed to Netlify automatically.
 To host or develop locally:
 
 ```
-pip install mkdocs mkdocs-material
+pip install mkdocs mkdocs-material markdown-include
 ```
 
 `mkdocs serve` and visit [localhost:8000](http://localhost:8000).