Update README.md

Adding docs status

Author: jbogard

README.md

@@ -3,6 +3,8 @@
 [![CI](https://github.com/automapper/automapper/workflows/CI/badge.svg)](https://github.com/AutoMapper/AutoMapper/actions?query=workflow%3ACI)
 [![NuGet](http://img.shields.io/nuget/vpre/AutoMapper.svg?label=NuGet)](https://www.nuget.org/packages/AutoMapper/)
 [![MyGet (dev)](https://img.shields.io/myget/automapperdev/vpre/AutoMapper.svg?label=MyGet)](https://myget.org/feed/automapperdev/package/nuget/AutoMapper)
+[![Documentation Status](https://readthedocs.org/projects/automapper/badge/?version=stable)](https://docs.automapper.org/en/stable/?badge=stable)
+
 
 ### What is AutoMapper?