add README badge

Author: slayoo

README.md

@@ -71,6 +71,8 @@ gas_data = ppmc.GasData(("H2SO4", "HNO3", "HCl", "NH3", "NO", "NO2"))
 [![nbviewer](https://raw.githubusercontent.com/jupyter/design/master/logos/Badges/nbviewer_badge.svg)](https://nbviewer.jupyter.org/github/open-atmos/PyPartMC/blob/main/examples/condense.ipynb)
 [![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/open-atmos/PyPartMC/blob/main/examples/condense.ipynb)
 [![Binder](https://mybinder.org/badge_logo.svg)](https://mybinder.org/v2/gh/open-atmos/PyPartMC.git/main?urlpath=lab/tree/examples/condense.ipynb)
+- `widgets_playground` notebook:   
+[![Binder](https://mybinder.org/badge_logo.svg)](https://mybinder.org/v2/gh/open-atmos/PyPartMC.git/main?urlpath=lab/tree/examples/widgets_playground.ipynb)
 
 #### usage in other projects