Add GitHub Actions build status badge to README

Author: dmadison

README.md

@@ -1,5 +1,5 @@
 # Arduino XInput Library 
-[![arduino-library-badge](https://www.ardu-badge.com/badge/XInput.svg?)](https://www.ardu-badge.com/XInput)
+[![arduino-library-badge](https://www.ardu-badge.com/badge/XInput.svg?)](https://www.ardu-badge.com/XInput) [![Build Status](https://github.com/dmadison/ArduinoXInput/workflows/build/badge.svg?branch=master)](https://github.com/dmadison/ArduinoXInput/actions?query=workflow%3Abuild)
 
 This library lets you easily emulate an Xbox 360 controller using a USB-capable Arduino microcontroller.