introduction text for Max OS X and Homebrew

Change-Id: Ifd8e6bbef0e6a0054306e30803b99da46847688f
initial docs on Xcode installation

Change-Id: Iae4a67b0bb6c4985083211c72de148c38c72d8ba
refinements of homebrew/xcode

Change-Id: I8e85d240a33a16e961cbb44732eed04a1666af90
fiddling with instruction layout

Change-Id: Ic4b6080f96f3d1b44bca6c803abc10bdfab620fa
minor edits

Change-Id: Iba3081d866d99ab28bbe69146bcf51189a018800
Almost done. But I need to talk about MACOSX_DEPLOYMENT_TARGET

Change-Id: I34c2500a8c6a66eff0792c7c2ef82f5e0d061742
trying to explain the complication of MACOSX_DEPLOYMENT_TARGET.

Will work on this later, again. Extremely hard to explain.
(and probably too much for the README.rst)

Change-Id: I659037e1ec2825b1453903915588a20196760892