doc(README): add codecov badge

Author: emptybutton

README.md

@@ -4,6 +4,7 @@
 [![GitHub Release](https://img.shields.io/github/v/release/emptybutton/ttt?style=flat&logo=github&labelColor=%23282e33&color=%237c73ff)](https://github.com/emptybutton/ttt/releases)
 [![Wakatime](https://wakatime.com/badge/user/0d3b7ff5-0547-4323-a43e-2a7308d973a0/project/f29e824e-5b14-420d-9f12-24f078225e48.svg)](https://wakatime.com/badge/user/0d3b7ff5-0547-4323-a43e-2a7308d973a0/project/f29e824e-5b14-420d-9f12-24f078225e48)
 [![Lines](https://img.shields.io/endpoint?url=https%3A%2F%2Fghloc.vercel.app%2Fapi%2Femptybutton%2Fttt%2Fbadge%3Ffilter%3D.py&logo=python&label=lines&color=blue)](https://github.com/search?q=repo%3Aemptybutton%2ttt+language%3APython+&type=code)
+[![Codecov](https://codecov.io/gh/emptybutton/ttt/graph/badge.svg?token=S5N9YYOZ4D)](https://codecov.io/gh/emptybutton/ttt)
 
 > ***T***ic-***T***ac-***T***oe