Update README to reflect dependencies

[lazerwalker]

A clean clone of the repo won't build, since it depends on https://github.com/soffes/x.

I'm not really a Carthage person, so I can't speak to whether the actual correct solution is to check in dependencies or not, but for now updating the README seems like a sensible quick fix.

[DivineDominion]

A heuristic I get by with: check in the code dependencies for your products, i.e. apps. But don't check them in for libraries.