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

diff --git a/BUILD_GUIDE.md b/BUILD_GUIDE.md
index d7106d91834e97c292595405b60f64950076f038..07ed0e98d730d40ae28351da248ca2a30b3ecfcd 100644 (file)
--- a/BUILD_GUIDE.md
+++ b/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