Add code-cov badge to README (#343)

Author: cmaglie

README.md

@@ -2,6 +2,7 @@
 
 [![Build Status](https://cloud.drone.io/api/badges/arduino/arduino-cli/status.svg)](https://cloud.drone.io/arduino/arduino-cli)
 [![Build status](https://ci.appveyor.com/api/projects/status/obclpbgsafum2wml/branch/master?svg=true)](https://ci.appveyor.com/project/Arduino/arduino-cli/branch/master)
+[![codecov](https://codecov.io/gh/arduino/arduino-cli/branch/master/graph/badge.svg)](https://codecov.io/gh/arduino/arduino-cli)
 
 `arduino-cli` is an all-in-one solution that provides builder, boards/library manager, uploader,
 discovery and many other tools needed to use any Arduino compatible board and platforms.