Codecov integration: readme

Author: horothesun

README.md

@@ -2,6 +2,7 @@
 
 ![CI Linux](https://github.com/horothesun/AsyncTaskSwitcher/workflows/CI%20Linux/badge.svg)
 ![CI macOS](https://github.com/horothesun/AsyncTaskSwitcher/workflows/CI%20macOS/badge.svg)
+![codecov](https://codecov.io/gh/horothesun/AsyncTaskSwitcher/branch/master/graph/badge.svg?token=6XPPUZBF4W)
 [![SwiftPM](https://img.shields.io/badge/SwiftPM-Compatible-brightgreen.svg)](https://swift.org/package-manager/)
 
 Swift asynchronous task switcher: it calls the completion handler of the most-recently-completed async task.