Update README.md with RTDs info

Author: astrochun

README.md

@@ -7,4 +7,7 @@ Set of common codes used in metallicity studies that use the stacking techniques
 [![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)
+![GitHub](https://img.shields.io/github/license/astrochun/Metallicity_Stack_Commons?color=blue)
+
+Documentation for this code is available on
+[Read the Docs](https://metallicity-stack-commons.readthedocs.io/en/latest/).