Added Mac section (copy/pasted from PCB_GUIDE.md)

BUILD_GUIDE.md

@@ -6,7 +6,17 @@
 What these instructions look like depends on whether NoahAndrews's setup scripts get merged
 
 ### Mac
+If you're using homebrew, you can use the following commands:
 
+    brew tap osx-cross/avr
+    brew install avr-libc
+    brew install dfu-programmer
+
+Otherwise, these instructions will work:
+
+1. Install Xcode from the App Store.
+2. Install the Command Line Tools from `Xcode->Preferences->Downloads`.
+3. Install [DFU-Programmer][dfu-prog].
 
 ### Linux