Mac Xcode installation instructions

[bobmyhill]

xcode-install is being sunset and replaced by xcodes (https://github.com/xcpretty/xcode-install/blob/master/MIGRATION.md).

One benefit to xcodes (https://github.com/XcodesOrg/xcodes) is that it can be run entirely on the command line, and seems to work a lot more smoothly than xcode-install does. It would be nice to change the user instructions to use the new tool.