Kickstart extensions are Git Submodules. Git Submodules are a neat way of including other git repositories in your own. You can keep them up-to-date with the main repo while not committing the entire contents of the repo with your project.
It's best to show how this works first, as there are some variations based on your environment and type of extension you'd like to install.
Rails users Using third-party components/themes is optional, but you'll need to do a few things first. See "Shimming rails" below before doing anything else.