Add a test coverage badge

Author: php1ic

README.md

@@ -1,5 +1,7 @@
 # Nuclear Masses
 
+[![Unit Tests](https://github.com/php1ic/nuclearmasses/actions/workflows/tests.yml/badge.svg)](https://github.com/php1ic/nuclearmasses/actions/workflows/tests.yml)
+
 ## Introduction
 
 Python package to parse the various files published by the [AME](https://www-nds.iaea.org/amdc/) and [NUBASE](http://amdc.in2p3.fr/web/nubase_en.html).