Add code coverage badge

Author: elgopher

README.md

@@ -2,6 +2,7 @@
 Generic functions to get optional values
 
 [![Go Reference](https://pkg.go.dev/badge/github.com/elgopher/ptr.svg)](https://pkg.go.dev/github.com/elgopher/ptr)
+[![codecov](https://codecov.io/gh/elgopher/ptr/branch/master/graph/badge.svg)](https://codecov.io/gh/elgopher/ptr)
 [![Project Status: Active – The project has reached a stable, usable state and is being actively developed.](https://www.repostatus.org/badges/latest/active.svg)](https://www.repostatus.org/#active)
 
 ## Problems with Go optional values