Add codecov badge URL

Author: joowani

README.md

@@ -2,6 +2,7 @@
 
 [![Build](https://github.com/ArangoDB-Community/python-arango/actions/workflows/build.yaml/badge.svg)](https://github.com/ArangoDB-Community/python-arango/actions/workflows/build.yaml)
 [![CodeQL](https://github.com/ArangoDB-Community/python-arango/actions/workflows/codeql.yaml/badge.svg)](https://github.com/ArangoDB-Community/python-arango/actions/workflows/codeql.yaml)
+[![codecov](https://codecov.io/gh/ArangoDB-Community/python-arango/branch/main/graph/badge.svg?token=M8zrjrzsUY)](https://codecov.io/gh/ArangoDB-Community/python-arango)
 [![PyPI version](https://badge.fury.io/py/python-arango.svg)](https://badge.fury.io/py/python-arango)
 [![GitHub license](https://img.shields.io/github/license/ArangoDB-Community/python-arango?color=brightgreen)](https://github.com/ArangoDB-Community/python-arango/blob/main/LICENSE)
 ![Python version](https://img.shields.io/badge/python-3.6%2B-blue)