Added note about using GNUmakefile

Board-STM32F407-Discovery.md

@@ -19,6 +19,13 @@ Cortex M4 STM32F407VG
    1. ```make BOARD=STM32F4DISC```
    1. ```export BOARD=STM32F4DISC``` in your environment and then just use make.
    1. Edit ```BOARD ?= PYBOARD4``` to read ```BOARD ?= STM32F4DISC```
+   1. Create a GNUmakefile GNUmakefile is not checked into git, so it will stay the same when you rebase. The info statements are optional, but I like to have them to remind me that I'm using a GNUmakefile. A simple GNUmakefile (note the lowercase m in makefile) would look something like the following:
+```
+$(info Executing GNUmakefile)
+BOARD = STM32F4DISC
+$(info BOARD = $(BOARD))
+include Makefile
+```
 
 ###Programming from Linux (via DFU)###
 This section covers how to flash the STM32F4DISCOVERY from linux (I was using Linux Mint 14, which is derived from ubuntu 12.10).