Docs: add codecov badge to readme

Author: raycadle

README.md

@@ -10,6 +10,8 @@ It is suitable for building custom tools or integrations around the Wallhaven AP
 > This project is unofficial and not affiliated with [Wallhaven.cc](https://wallhaven.cc).
 > Use at your own discretion and in accordance with the site's [terms of service](https://wallhaven.cc/about).
 
+[![codecov](https://codecov.io/gh/raycadle/WallhavenAPI/graph/badge.svg?token=9V91WO5EBN)](https://codecov.io/gh/raycadle/WallhavenAPI)
+
 [Coverage Reports](https://raycadle.github.io/WallhavenAPI/coverage)
 
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