Update: add codecov badge

Author: heptaflar

README.md

@@ -4,7 +4,7 @@ Monte Carlo uncertainty propagation for [ESA SCOPE](https://oceancarbon-scope.or
 
 [![Package](https://github.com/bcdev/kaleidoscope/actions/workflows/python-package.yml/badge.svg)](https://github.com/bcdev/kaleidoscope/actions/workflows/python-package.yml)
 [![CodeQL Advanced](https://github.com/bcdev/kaleidoscope/actions/workflows/codeql.yml/badge.svg)](https://github.com/bcdev/kaleidoscope/actions/workflows/codeql.yml)
-
+[![codecov](https://codecov.io/github/bcdev/kaleidoscope/graph/badge.svg?token=U4cOHk1I0t)](https://codecov.io/github/bcdev/kaleidoscope)
 
 ## Installing and testing