add Appveyor badge

Author: masci

Diff for: README.md

@@ -1,6 +1,7 @@
 # arduino-cli
 
 [![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/d5ga79b8uifijas9/branch/master?svg=true)](https://ci.appveyor.com/project/Arduino/arduino-cli/branch/master)
 
 `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.