Add Arduino CLI compilation to docs

Author: nseidle

docs/firmware_update.md

@@ -479,3 +479,7 @@ Enable the libraries in the Arduino IDE
 
 35. cd ~/SparkFun/RTK/
 36. cp  Firmware/app3M_fat9M_16MB.csv  ~/.arduino15/packages/esp32/hardware/esp32/2.0.2/tools/partitions/app3M_fat9M_16MB.csv
+
+### Arduino CLI
+
+The firmware can be compiled using [Arduino CLI](https://github.com/arduino/arduino-cli). This makes compilation fairly platform independent and flexible. All release candidates and firmware releases are compiled using Arduino CLI using a github action. You can see the source of the action [here](https://github.com/sparkfun/SparkFun_RTK_Firmware/blob/main/.github/workflows/compile-release.yml), and use it as a starting point for Arduino CLI compilation.