docs: added codecov badge to README

Author: ErikBjare

README.md

@@ -3,6 +3,7 @@
 > A webui for ActivityWatch built in Vue.js
 
 [![Build Status](https://github.com/ActivityWatch/aw-webui/workflows/Build/badge.svg)](https://github.com/ActivityWatch/aw-webui/actions)
+[![Coverage Status](https://codecov.io/gh/ActivityWatch/aw-webui/branch/master/graph/badge.svg)](https://codecov.io/gh/ActivityWatch/aw-webui)
 [![Known Vulnerabilities](https://snyk.io/test/github/ActivityWatch/aw-webui/badge.svg)](https://snyk.io/test/github/ActivityWatch/aw-webui)
 
 ## Getting started