add codecov badge

Author: franckgaga

README.md

@@ -1,6 +1,7 @@
 # ModelPredictiveControl.jl
 
 [![Build Status](https://github.com/franckgaga/ModelPredictiveControl.jl/actions/workflows/CI.yml/badge.svg?branch=main)](https://github.com/franckgaga/ModelPredictiveControl.jl/actions/workflows/CI.yml?query=branch%3Amain)
+[![codecov](https://codecov.io/gh/franckgaga/ModelPredictiveControl.jl/branch/main/graph/badge.svg?token=K4V0L113M4)](https://codecov.io/gh/franckgaga/ModelPredictiveControl.jl)
 [![doc-stable](https://img.shields.io/badge/docs-stable-blue.svg)](https://franckgaga.github.io/ModelPredictiveControl.jl/stable)
 [![coc-dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://franckgaga.github.io/ModelPredictiveControl.jl/dev)