Add contributing info to README

Author: mfisher87

README.md

@@ -1 +1,7 @@
 # JupyterCon 2025 Workshop: Extension Development for Everyone!
+
+
+## Contributing
+
+This repository uses [`pixi`](https://pixi.sh) (at least for now).
+To start up a website preview server, run `pixi run preview`.