Improve documentation around running bootstrap first

My colleagues and I just ran into an issue where we didn't realised that bootstrap must be run before Carthage will build the dependencies correctly.

When running build on a project that contained the full source in the Checkouts folder, ReactiveCocoa complained about not being able to import Result, one of the dependencies, and the project failed to build.

When we realised we might need to use bootstrap, the project build successfully. It'd be great to improve the documentation around this.