Updated Appendix C: Firmware programming (markdown)

Appendix-C:-Firmware-programming.md

@@ -69,6 +69,10 @@ If the Arduino IDE reports that you successfully flashed the firmware onto the A
 In this step, you'll use the Arduino Nano to program the Ploopy Trackball.
 
 1. Visit the [Ploopy Trackball Github firmware page](https://github.com/ploopyco/mouse/tree/master/firmware) and download the entire `production` directory onto your computer.
+   - *NOTE THAT* not all of the firmware versions will work on all of the boards!
+   - First, check to see what kind of main board you have. Look at the board for a revision number (the board containing the sensor, the one that lies flat, not the vertical one). The code should look something like "R1.00X", where X is some number.
+   - If that number is R1.004, R1.003 or, very rarely, R1.002, use the firmware in the ["r1.004" branch of the code](https://github.com/ploopyco/trackball/tree/r1.004).
+   - If R1.005 or higher, use the [main branch](https://github.com/ploopyco/trackball/tree/master).
 2. If it's not running, start the Arduino IDE.
 3. Open the Ploopy firmware by selecting `File -> Open` and selecting `production.ino`.
 4. Ensure that a 10uF capacitor is placed between RST and GND on the Arduino Nano. (The polarity of the capacitor doesn't matter. So, if you're using an [electrolytic capacitor](https://media.rs-online.com/t_large/F5194059-01.jpg), it doesn't matter which way you plug it in.)