Add coveralls.io coverage to README.md

Author: krishnans2006

README.md

@@ -9,6 +9,7 @@
   <a href="https://github.com/astral-sh/ruff"><img src="https://img.shields.io/endpoint?url=https://raw.githubusercontent.com/astral-sh/ruff/main/assets/badge/v2.json"></a>
   <img src="https://github.com/tjcsl/tin/actions/workflows/ci.yml/badge.svg">
   <a href="https://github.com/pre-commit/pre-commit"><img src="https://img.shields.io/badge/pre--commit-enabled-brightgreen?logo=pre-commit"></a>
+  <a href='https://coveralls.io/github/tjcsl/tin'><img src='https://coveralls.io/repos/github/tjcsl/tin/badge.svg' alt='Coverage Status' /></a>
   <a href="https://wakatime.com/badge/github/tjcsl/tin"><img src="https://wakatime.com/badge/github/tjcsl/tin.svg" alt="wakatime"></a>
 </p>
 <hr>