Add Receptor Testing pipeline status badge

Author: elyezer

README.md

@@ -2,6 +2,7 @@ Receptor
 ==============
 
 [![Documentation](https://readthedocs.org/projects/receptor/badge/?version=stable)](https://receptor.readthedocs.io/en/latest/)
+[![Receptor Testing](https://github.com/project-receptor/receptor/workflows/Receptor%20Testing/badge.svg)](https://github.com/project-receptor/receptor/actions?query=workflow%3A%22Receptor+Testing%22)
 
 
 For the latest documentation see: [https://receptor.readthedocs.io](https://receptor.readthedocs.io/en/latest/)