add contributing

CONTRIBUTING.md

@@ -25,3 +25,18 @@ Please make sure you have test coverage for anything you add or fix!
 Please add a CHANGELOG entry with any relevant tags for issues, pull-requests, and authors.
 
 Thanks for you contribution!
+
+### Modify the Documentation
+
+The documentation is written in Markdown and is located in the `docs` directory. The documentation is built using [VitePress](https://vitepress.dev).
+
+```bash
+npm install
+npm run dev
+
+# build for production, resulting in a static site in docs/.vitepress/dist
+npm run build
+```
+
+Before submitting a pull request, please make sure the documentation builds correctly using `npm run build`.
+Most likely the build will fail if there are any syntax errors in the markdown files or dead links.