Update Readme: serve locally

Author: DavidMStraub

README.md

@@ -10,6 +10,22 @@ For the source code of Gramps Web, please see
 Please use those repositories for reporting issues or feature requests.
 
 
+## Serving the documentation locally
+
+You can run the website locally and preview it in your browser while editing.
+
+To do that, first install the required Python dependencies (you may want to do this in a virtual environment):
+
+```bash
+python -m pip install -r requirements.txt
+```
+
+Then, you can serve the documentation and open it in the browser with:
+
+```bash
+mkdocs serve -a localhost:8211 -o
+```
+
 ## Contributing
 
 Contributions to the documentation are welcome! Please see the [Contributing Guide](CONTRIBUTING.md) for details.