add doc badge to README

Author: Remi-Gau

README.md

@@ -4,6 +4,10 @@
 
 [![Binder](https://mybinder.org/badge_logo.svg)](https://mybinder.org/v2/gh/cpp-lln-lab/CPP_BIDS/master?filepath=notebooks%2Fbasic_usage.ipynb)
 
+**Documentation**
+
+[![Documentation Status: stable](https://readthedocs.org/projects/cpp-bids/badge/?version=stable)](https://cpp-bids.readthedocs.io/en/stable/?badge=stable)
+
 **Cite it**
 
 [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.4007674.svg)](https://doi.org/10.5281/zenodo.4007674)