Carthage 0.3 adds a new --use-submodules flag to the bootstrap, update, and checkout commands, allowing you to add dependencies as Git submodules, instead of simply checking out their source code into your project's directory.
This is intended to make framework development easier, as well as provide a way to preserve dependencies' version history without polluting your project's history.
For more information, please see the section in the README or consult carthage help.
This release also includes a major fix for a Cartfile.lock ordering bug that can result in failed builds.