misc: add codecov badge

Author: Taxrosdev

.gitignore

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 /target
 example_*
-tarpaulin-report.html

README.md

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 # Linked Content Addressed Storage
 
-[![Coverage Status](https://coveralls.io/repos/github/TimelessOS/LCAS/badge.svg?branch=main)](https://coveralls.io/github/TimelessOS/LCAS?branch=main)
+[![codecov](https://codecov.io/gh/TimelessOS/LCAS/graph/badge.svg?token=5W9K4IK81O)](https://codecov.io/gh/TimelessOS/LCAS)
 
 A simple yet complex method of storing large directory structures with duplicate files