Add ActionCI badge

Author: SergeStinckwich

README.md

@@ -13,6 +13,7 @@
 </p>
 
 [![Pharo version](https://img.shields.io/badge/Pharo-8.0-%23aac9ff.svg)](https://pharo.org/download)
+[![CI matrix](https://github.com/PolyMathOrg/PolyMath/actions/workflows/smalltalk-ci.yml/badge.svg)](https://github.com/PolyMathOrg/PolyMath/actions/workflows/smalltalk-ci.yml)
 [![Coverage Status](https://coveralls.io/repos/github/PolyMathOrg/PolyMath/badge.svg?branch=master)](https://coveralls.io/github/PolyMathOrg/PolyMath?branch=master)
 [![License](https://img.shields.io/badge/license-MIT-blue.svg)](https://raw.githubusercontent.com/PolyMathOrg/PolyMath/master/LICENSE)