Add pypi badge (#10)

Co-authored-by: Frédéric Collonval <[email protected]>

Author: Frédéric Collonval

README.md

@@ -7,6 +7,7 @@
 # jupyter_nbmodel_client
 
 [![Github Actions Status](https://github.com/datalayer/jupyter-nbmodel-client/workflows/Build/badge.svg)](https://github.com/datalayer/jupyter-nbmodel-client/actions/workflows/build.yml)
+[![PyPI - Version](https://img.shields.io/pypi/v/jupyter-nbmodel-client)](https://pypi.org/project/jupyter-nbmodel-client)
 
 Client to interact with Jupyter notebook model.