Add badges (#105)

Author: fcollonval

README.md

@@ -1,5 +1,7 @@
 [![Build Status](https://github.com/jupyter-server/jupyter_ydoc/workflows/Tests/badge.svg)](https://github.com/jupyter-server/jupyter_ydoc/actions)
 [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
+[![PyPI](https://img.shields.io/pypi/v/jupyter-ydoc)](https://pypi.org/project/jupyter-ydoc/)
+[![npm (scoped)](https://img.shields.io/npm/v/@jupyter/ydoc)](https://www.npmjs.com/package/@jupyter/ydoc)
 
 # jupyter_ydoc