docs: add code-coverage badge to readme

Author: Jost Voelker

README.md

@@ -1,5 +1,7 @@
 # Installation
 
+[![codecov](https://codecov.io/gh/brainbits/database-command/branch/main/graph/badge.svg?token=DQUKMP6AF5)](https://codecov.io/gh/brainbits/database-command)
+
 Register the commands in your `services.yaml`.
 
 ```yaml