chore(readme): add codecov badge

README.md

@@ -8,6 +8,7 @@
 [![Docker Pulls](https://badgen.net/docker/pulls/meienberger/runtipi?icon=docker&label=pulls)](https://hub.docker.com/r/meienberger/runtipi/)
 [![Docker Image Size](https://badgen.net/docker/size/meienberger/runtipi?icon=docker&label=image%20size)](https://hub.docker.com/r/meienberger/runtipi/)
 ![Build](https://github.com/meienberger/runtipi/workflows/Tipi%20CI/badge.svg)
+[![codecov](https://codecov.io/gh/meienberger/runtipi/branch/master/graph/badge.svg?token=FZGO7ZOPSF)](https://codecov.io/gh/meienberger/runtipi)
 #### Join the discussion
 [![Discord](https://img.shields.io/discord/976934649643294750?label=discord&logo=discord)](https://discord.gg/Bu9qEPnHsc)
 [![Matrix](https://img.shields.io/matrix/runtipi:matrix.org?label=matrix&logo=matrix)](https://matrix.to/#/#runtipi:matrix.org)