add dfu_flash documentation (#527)

Author: nerdCopter

docs/development/building/Dfu_Flash.md

@@ -0,0 +1,19 @@
+# Building DFU_Flash
+
+Pre-requisites: python3, python3-intelhex
+
+### Make and Flash via dfu-util:
+
+For Betaflight 4.5 and later use the command syntax:
+
+```
+make CONFIG=TARGETNAME dfu_flash
+```
+
+Where `TARGETNAME` is a specified valid flight-controller target.
+
+Example:
+
+```
+make CONFIG=MATEKF405AIO dfu_flash
+```