Add instructions for zadig when bootloader device is unknown

docs/faq_build.md

@@ -17,7 +17,7 @@ or just:
 
 Note that running `make` with `sudo` is generally *not* a good idea, and you should use one of the former methods, if possible.
 
-## Linux `udev` Rules
+### Linux `udev` Rules
 On Linux, you'll need proper privileges to access the MCU. You can either use
 `sudo` when flashing firmware, or place these files in `/etc/udev/rules.d/`.
 
@@ -37,6 +37,14 @@ SUBSYSTEMS=="usb", ATTRS{idVendor}=="03eb", ATTRS{idProduct}=="2ff0", MODE:="066
 SUBSYSTEMS=="usb", ATTRS{idVendor}=="feed", MODE:="0666"
 ```
 
+## Unknown Device for DFU Bootloader
+
+If you're using Windows to flash your keyboard, and you are running into issues, check the Device Manager.  If you see an "Unknown Device" when the keyboard is in "bootloader mode", then you may have a driver issue. 
+
+Re-running the installation script for MSYS2 may help (eg run `./util/qmk_install.sh` from MSYS2/WSL) or reinstalling the QMK Toolbox may fix the issue. 
+
+If that doesn't work, then you may need to grab the [Zadig Utility](https://zadig.akeo.ie/). Download this, find the device in question, and select the `WinUS(libusb-1.0)` option, and hit "Reinstall driver". Once you've done that, try flashing your board, again. 
+
 ## WINAVR is Obsolete
 It is no longer recommended and may cause some problem.
 See [TMK Issue #99](https://github.com/tmk/tmk_keyboard/issues/99).