intall GDAL on MAC.md

from http://www.davidlemayian.com/post/60791205171/how-to-install-python-gdal-on-os-x

intall gdal first and then install GDAL for python

$ brew install gdal $ sudo pip install GDAL

if GDAL is has cc error, then it is due to xcode5.1, just install GDAL with

ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future ./venv/bin/pip install GDAL