readme: added coverage badge

README.md

@@ -2,6 +2,7 @@
 
 [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
 [![CI Pipeline Status](https://github.com/fphammerle/ical2vdir/workflows/tests/badge.svg)](https://github.com/fphammerle/ical2vdir/actions)
+[![Coverage Status](https://coveralls.io/repos/github/fphammerle/ical2vdir/badge.svg?branch=master)](https://coveralls.io/github/fphammerle/ical2vdir?branch=master)
 [![Last Release](https://img.shields.io/pypi/v/ical2vdir.svg)](https://pypi.org/project/ical2vdir/#history)
 [![Compatible Python Versions](https://img.shields.io/pypi/pyversions/ical2vdir.svg)](https://pypi.org/project/ical2vdir/)