Add code coverage badge

Author: nhooyr

README.md

@@ -1,6 +1,7 @@
 # websocket
 
 [![GoDoc](https://godoc.org/nhooyr.io/websocket?status.svg)](https://godoc.org/nhooyr.io/websocket)
+[![Codecov](https://img.shields.io/codecov/c/github/nhooyr/websocket.svg?color=brightgreen)](https://codecov.io/gh/nhooyr/websocket)
 
 websocket is a minimal and idiomatic WebSocket library for Go.