Groundwork, like Bootstrap, is a web-development framework, not just a set of CSS docs. They both include a set of style-sheets, java-script files, misc icons etc, and one or more HTML docs to bring it all together.Installation is simple; download the framework, rename the folder to "MyCoolNewSite", start editing.
As Michal stated, the "index/html" doc that comes with the framework has lots to look at to help you get started. They also have what looks to be pretty solid documentation on their site, starting here: http://groundwork.sidereel.com/?url=grid
The site has now changed to: http://groundworkcss.github.io/