docs: Add Codecov coverage badge to README

Add Codecov badge to the badge section showing test coverage
percentage with link to detailed coverage reports.

Author: github-actions

README.md

@@ -3,6 +3,7 @@
 [![Tests](https://github.com/Abstract-Data/RyanData-Address-Utils/actions/workflows/tests.yml/badge.svg)](https://github.com/Abstract-Data/RyanData-Address-Utils/actions/workflows/tests.yml)
 [![Ruff](https://github.com/Abstract-Data/RyanData-Address-Utils/actions/workflows/lint.yml/badge.svg)](https://github.com/Abstract-Data/RyanData-Address-Utils/actions/workflows/lint.yml)
 [![MyPy](https://github.com/Abstract-Data/RyanData-Address-Utils/actions/workflows/typecheck.yml/badge.svg)](https://github.com/Abstract-Data/RyanData-Address-Utils/actions/workflows/typecheck.yml)
+[![codecov](https://codecov.io/gh/Abstract-Data/RyanData-Address-Utils/graph/badge.svg?token=75LQK4KJTZ)](https://codecov.io/gh/Abstract-Data/RyanData-Address-Utils)
 [![Python 3.9+](https://img.shields.io/badge/python-3.9+-blue.svg)](https://www.python.org/downloads/)
 [![uv](https://img.shields.io/badge/packaging-uv-9055ff.svg)](https://github.com/astral-sh/uv)
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)