add coverage badge to readme.

README.md

@@ -3,6 +3,7 @@
 [![](https://img.shields.io/badge/read-the_docs-brightgreen)](https://pycodegen.pages.i10git.cs.fau.de/pystencils-sfg)
 [![](https://i10git.cs.fau.de/pycodegen/pystencils-sfg/badges/master/pipeline.svg)](https://i10git.cs.fau.de/pycodegen/pystencils-sfg/commits/master)
 [![](https://img.shields.io/gitlab/license/pycodegen%2Fpystencils-sfg?gitlab_url=https%3A%2F%2Fi10git.cs.fau.de)](https://i10git.cs.fau.de/pycodegen/pystencils-sfg/-/blob/master/LICENSE)
+![coverage](https://i10git.cs.fau.de/pycodegen/pystencils-sfg/badges/master/coverage.svg)
 
 
 A bridge over the semantic gap between code emitted by pystencils and your C/C++/Cuda/HIP framework.