boards: add flashing info using dfu-util for STM32F405

Signed-off-by: deadprogram <ron@hybridgroup.com>

Author: deadprogram

content/microcontrollers/feather-stm32f405.md

@@ -22,21 +22,35 @@ The [Adafruit Feather STM32F405](https://www.adafruit.com/product/4382) is a tin
 
 ## Flashing
 
+Flashing this board using its DFU bootloader can be a bit cumbersome (see Adafruit docs at https://learn.adafruit.com/adafruit-stm32f405-feather-express/dfu-bootloader-details), but it is possible - without requiring an external programmer - by pulling the B0 pin high (you can use the board's 3.3V output pin) at bootup when connected to host PC via USB. This puts the device in bootloader mode.
+
+Once in bootloader mode, the device can be programmed using the open-source tool `dfu-util`.
+
 ### CLI Flashing on Linux
 
-Goes here
+You must first install the `dfu-util` program in order to flash the Adafruit Feather STM32F405 board.
+
+    sudo apt update 
+    sudo apt install dfu-util
 
 ### CLI Flashing on macOS
 
-Goes here
+You must first install the `dfu-util` program in order to flash the Adafruit Feather STM32F405 board. You can do this using Homebrew on macOS:
+
+    brew install dfu-util
 
 ### CLI Flashing on Windows
 
-Goes here
+You must first install the `dfu-util` program in order to flash the Adafruit Feather STM32F405 board.
+
+- Download dfu-util from the website here: http://dfu-util.sourceforge.net/releases/dfu-util-0.9-win64.zip
+- Decompress the files to a directory such as `C:\dfu-util`
+- Add `C:\dfu-util` to your `PATH`.
 
 ### Troubleshooting
 
-Goes here
+If you run into trouble getting dfu-util installed and working on Windows, see the blog post at https://www.hanselman.com/blog/HowToFixDfuutilSTMWinUSBZadigBootloadersAndOtherFirmwareFlashingIssuesOnWindows.aspx
+
 
 ## Notes