Update README.md

Co-authored-by: Michael Schlottke-Lakemper <[email protected]>

README.md

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 # TrixiBase.jl
 
+[![Docs-stable](https://img.shields.io/badge/docs-stable-blue.svg)](https://trixi-framework.github.io/TrixiBase.jl/stable)
+[![Docs-dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://trixi-framework.github.io/TrixiBase.jl/dev)
+[![Build Status](https://github.com/trixi-framework/TrixiBase.jl/workflows/CI/badge.svg)](https://github.com/trixi-framework/TrixiBase.jl/actions?query=workflow%3ACI)
+[![Coveralls](https://coveralls.io/repos/github/trixi-framework/TrixiBase.jl/badge.svg)](https://coveralls.io/github/trixi-framework/TrixiBase.jl)
+[![Codecov](https://codecov.io/gh/trixi-framework/TrixiBase.jl/branch/main/graph/badge.svg)](https://codecov.io/gh/trixi-framework/TrixiBase.jl)
+[![License: MIT](https://img.shields.io/badge/License-MIT-success.svg)](https://opensource.org/license/mit/)
+
 TrixiBase.jl provides common functionality used by multiple Julia packages in the
 [Trixi Framework](https://github.com/trixi-framework).