document missing dependency

README.md

@@ -52,7 +52,7 @@ To test it locally:
 1. Install the required tools (See also [.github/workflows/build-docs.yaml](.github/workflows/build-docs.yaml)):
     ```
     pip install --upgrade pip
-    pip install mkdocs mkdocs-gen-files mkdocs-awesome-pages-plugin mkdocs-material pymdown-extensions
+    pip install mkdocs mkdocs-gen-files mkdocs-awesome-pages-plugin mkdocs-material pymdown-extensions mkdocs-enumerate-headings-plugin
     ```
 1. In the main folder of the repo, call `mkdocs serve` (and keep it running).
   This will locally generate the documentation.