add badge for documentation

Author: RRosio

README.md

@@ -1,7 +1,7 @@
 # Jupyter Notebook as a Jupyter Server Extension
 
 ![Testing nbclassic](https://github.com/jupyterlab/nbclassic/workflows/Testing%20nbclassic/badge.svg)
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+[![Documentation Status](https://readthedocs.org/projects/nbclassic/badge/?version=latest)](https://nbclassic.readthedocs.io/en/latest/?badge=latest)
 
 NBClassic runs the [Jupyter NbClassic](https://github.com/jupyter/nbclassic) frontend on the Jupyter Server backend.