change Readme

Author: tknopp

README.md

@@ -1,3 +1,5 @@
 # LinearOperatorCollection
 
 [![Build Status](https://github.com/JuliaImageRecon/LinearOperatorCollection.jl/actions/workflows/CI.yml/badge.svg?branch=main)](https://github.com/JuliaImageRecon/LinearOperatorCollection.jl/actions/workflows/CI.yml?query=branch%3Amain)
+
+[![codecov.io](http://codecov.io/JuliaImageRecon/LinearOperatorCollection.jl/coverage.svg?branch=master)](http://codecov.io/github/JuliaImageRecon/LinearOperatorCollection.jl?branch=master)