Add Codecov coverage badge to README (#103)

Author: GeiserX

README.md

@@ -13,6 +13,7 @@
   <a href="https://github.com/GeiserX/Telegram-Archive/stargazers"><img src="https://img.shields.io/github/stars/GeiserX/Telegram-Archive?style=flat-square&logo=github" alt="GitHub Stars"></a>
   <a href="LICENSE"><img src="https://img.shields.io/github/license/GeiserX/Telegram-Archive?style=flat-square" alt="License"></a>
   <a href="https://github.com/GeiserX/Telegram-Archive/releases"><img src="https://img.shields.io/github/v/release/GeiserX/Telegram-Archive?style=flat-square" alt="Release"></a>
+  <a href="https://codecov.io/gh/GeiserX/Telegram-Archive"><img src="https://codecov.io/gh/GeiserX/Telegram-Archive/graph/badge.svg" alt="codecov"></a>
 </p>
 
 <p align="center">