chore(docs): add Codecov badge

readme.md

@@ -4,6 +4,7 @@
 [![Docker Pulls](https://img.shields.io/docker/pulls/flagsmith/flagsmith)](https://hub.docker.com/u/flagsmith)
 [![Docker Image Size](https://img.shields.io/docker/image-size/flagsmith/flagsmith)](https://hub.docker.com/r/flagsmith/flagsmith)
 [![Join the Discord chat](https://img.shields.io/discord/517647859495993347)](https://discord.gg/hFhxNtXzgm)
+[![Coverage](https://codecov.io/gh/Flagsmith/flagsmith/branch/master/graph/badge.svg?token=IyGii7VSdc)](https://codecov.io/gh/Flagsmith/flagsmith)
 
 [Flagsmith](https://flagsmith.com/) is an open source, fully featured, Feature Flag and Remote Config service. Use our
 hosted API, deploy to your own private cloud, or run on-premise.