Zephyr comes in two parts; the main zephyr project and a sample project. You will need both to get started (the sample project is an excellent starter project).
$ git clone https://github.com/Sotera/zephyr.git
$ git clone https://github.com/Sotera/zephyr-sample-project.git
This will clone both the zephyr project and the sample project. While you don't need the sample project to use zephyr, it's still good to have it as a frame of reference.