Add a coverage badge to the readme file.

Note regarding implementation:

• That is the workflow of the coverage report: https://github.com/MishaKav/pytest-coverage-comment/blob/main/.github/workflows/live-test.yml
• https://github.com/marketplace/actions/dynamic-badges describes how to prepare everything for a dynamic badge (this is the dependency of pytest-coverage-comment): create a gist, make it public, create a secret for that gist, add it to the repository
• adjust the first script with the information of the second: gist id, file_name, secret name
• Important: You have to use the "githubusercontent" URL to work, see Domain is blocked [github.com] badges/shields#7435