Add RTDs badge in README.md #106

Author: astrochun

README.md

@@ -4,6 +4,7 @@ Set of common codes used in metallicity studies that use the stacking techniques
 [![GitHub Workflow Status (master)](https://img.shields.io/github/workflow/status/astrochun/Metallicity_Stack_Commons/Python%20package/master?color=blue&label=build%20%28master%29&logo=github)](https://github.com/astrochun/Metallicity_Stack_Commons/actions?query=workflow%3A%22Python+package%22+branch%3Amaster)
 [![GitHub Workflow Status](https://img.shields.io/github/workflow/status/astrochun/Metallicity_Stack_Commons/Python%20package?color=blue&label=build%20%28latest%29&logo=github)](https://github.com/astrochun/Metallicity_Stack_Commons/actions?query=workflow%3A%22Python+package%22)
 [![GitHub Workflow Status](https://img.shields.io/github/workflow/status/astrochun/Metallicity_Stack_Commons/Sphinx%20Docs%20Check?label=docs&color=blue)](https://github.com/astrochun/Metallicity_Stack_Commons/actions?query=workflow%3A%22Sphinx+Docs+Check%22)
+[![Read the Docs](https://img.shields.io/readthedocs/metallicity-stack-commons?color=blue&label=RTDs)](https://readthedocs.org/projects/metallicity-stack-commons/builds/)
 ![GitHub top language](https://img.shields.io/github/languages/top/astrochun/Metallicity_Stack_Commons)
 ![GitHub release (latest by date)](https://img.shields.io/github/v/release/astrochun/Metallicity_Stack_Commons)
 ![GitHub](https://img.shields.io/github/license/astrochun/Metallicity_Stack_Commons?color=blue)