Add XCode Dependency to session 00 instructions

[nuclearsandwich]

I blew away XCode on my machine to test the instructions but apparently I didn't blow hard enough. We do need XCode for our Apple participants. Luckily, they can install it while they wait for the instructor to build provisioning profiles. Having XCode does simplify the provisioning profile installation since XCode will place it in that weird ~/Library/MobileDevice/.. folder.

We can also distribute the Certificate file and Provisioning profile through a published url. I would like to keep SV specifics out of this repository as much as possible but that may not be an option.

[nuclearsandwich]

Here's a first crack at XCode instructions. https://gist.github.com/nuclearsandwich/6719141