Add homebrew build environment setup

From discussion [here](zsa/docs#2) it became clear that this information should be more available.

keyboard/ergodox_ez/README.md

@@ -15,7 +15,7 @@ This requires a little bit of familiarity with coding.
 
 1. Go to https://github.com/jackhumbert/qmk_firmware and read the Readme at the base of this repository, top to bottom. Then come back here :)
 2. Clone the repository (download it)
-3. Set up a build environment as per https://www.pjrc.com/teensy/gcc.html
+3. [Set up a build environment](https://www.pjrc.com/teensy/gcc.html) (on a mac and have [homebrew](http://brew.sh/)? just run `brew tap osx-cross/avr && brew install avr-libc`)
 4. Copy `keymaps/keymap_default.c` into `keymaps/keymaps_your_name.c` (for example, `keymaps/keymap_german.c`)
 5. Edit this file, changing keycodes to your liking (see "Finding the keycodes you need" below). Try to edit the comments as well, so the "text graphics" represent your layout correctly.
 6. Compile your firmware by running `make clean` followed by `make KEYMAP=your_name`. Note that you must omit the `keymap_` prefix for your filename in this command -- for example, `make KEYMAP=german`. This will result in a hex file, which will always be called `ergodox_ez.hex`, regardless of your keymap name.