Added codecov badge to README (#33)

Author: timtreis

README.md

@@ -2,10 +2,13 @@
 
 [![Tests][badge-tests]][link-tests]
 [![Documentation][badge-docs]][link-docs]
+[![Codecov][badge-codecov]][link-codecov]
 
 [badge-tests]: https://img.shields.io/github/actions/workflow/status/scverse/spatialdata-plot/test.yaml?branch=main
 [link-tests]: https://github.com/scverse/spatialdata-plot/actions/workflows/test.yml
 [badge-docs]: https://img.shields.io/readthedocs/spatialdata-plot
+[badge-codecov]: https://codecov.io/gh/scverse/spatialdata-plot/branch/main/graph/badge.svg?token=C45F3ATSVI
+[link-codecov]: https://app.codecov.io/gh/scverse/spatialdata-plot
 
 A very interesting piece of code