Add build status badge to README.md

Author: bvernoux

README.md

@@ -3,6 +3,8 @@ firmware
 
 HydraSDR RFOne Firmware (called also HydraSDR rfone_fw)
 
+[![Build HydraSDR RFOne Firmware](https://github.com/hydrasdr/rfone_fw/actions/workflows/build.yml/badge.svg)](https://github.com/hydrasdr/rfone_fw/actions/workflows/build.yml)
+
 * [HydraSDR-RFOne firmware directory structure and software architecture](https://github.com/hydrasdr/rfone_fw/blob/master/HydraSDR-RFOne-firmware-directory-structure-and-software-architecture.md)
 * [How to check the firmware version](https://github.com/hydrasdr/rfone_fw/blob/master/How-to-check-the-firmware-version.md)
 * How To Windows: