Add page for contributing to the docs (#4562)

docs/README.md

@@ -0,0 +1,21 @@
+# Documentation
+
+This is the home of the new documentation for uv.
+
+The documentation is a work in progress and not ready for external references.
+
+## Contributing
+
+Install mdbook:
+
+```
+cargo install mdbook
+```
+
+To run the documentation locally:
+
+```
+mdbook watch docs --open
+```
+
+The documentation is published to GitHub Pages on merge to `main`.