This repository contains the contents of <JPATH_SITE>/plugins/system/one.
Assuming you have a repository for your site, use these commands:
$ cd <JPATH_SITE>
$ git submodule add https://github.com/pdelbar/one-content-system-plugin.git plugins/system/one
$ git submodule init
$ git submodule update
Make sure to commit the .gitmodules file. Working with modules is explained here.