Update README.md

Author: Alexander-Barth

README.md

@@ -1,5 +1,5 @@
 [![Build Status](https://github.com/JuliaGeo/CommonDataModel.jl/workflows/CI/badge.svg)](https://github.com/JuliaGeo/CommonDataModel.jl/actions)
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+[![codecov](https://codecov.io/github/JuliaGeo/CommonDataModel.jl/graph/badge.svg?token=TNU4HSPelE)](https://codecov.io/github/JuliaGeo/CommonDataModel.jl)
 [![documentation stable](https://img.shields.io/badge/docs-stable-blue.svg)](https://juliageo.github.io/CommonDataModel.jl/stable/)
 [![documentation dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://juliageo.github.io/CommonDataModel.jl/dev/)