Add quick setup guide to CONTRIBUTING notes for developers

When I cloned my fork to work on #751, I opened the Xcode project, hit build and watched it fail completely.

It was only digging through the README that I found the reference to `script/bootstrap`, hidden away in the "Importing ReactiveCocoa" section.

It would be clearer to have some quick setup steps for new contributors in the CONTRIBUTING.md file as this is the first place I looked.