faq_build.md: Add section about serial devices on Linux

docs/faq_build.md

@@ -37,6 +37,10 @@ SUBSYSTEMS=="usb", ATTRS{idVendor}=="03eb", ATTRS{idProduct}=="2ff0", MODE:="066
 SUBSYSTEMS=="usb", ATTRS{idVendor}=="feed", MODE:="0666"
 ```
 
+### Serial device is not detected in bootloader mode on Linux
+Make sure your kernel has appropriate support for your device. If your device uses USB ACM, such as 
+Pro Micro (Atmega32u4), make sure to include `CONFIG_USB_ACM=y`. Other devices may require `USB_SERIAL` and any of its sub options.
+
 ## 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.