The builds branch contains both pre-built hex files as well as assembled source files generated by the configs in the master branch of this repo.
The release directory contains pre-compiled hex files for each of the configs defined in the master branch configs directory.
These files may be uploaded directly to any ATMega328 based hardware using a tool such as XLoader or via avrdude directly.
The code directory contains the build sources based on the modifications in the configs directory in the master branch. When the Travis CI runs, these directories are automatically created based on that folder structure and builds the pre-compiled hex files mentioned above.
These aggregates sources are provided so that you can use them as a starting point or build and upload directly from the Arduino IDE. Simply clone this repo and switch to the builds branch or download this branch's Zip file then browse to the code directory.
From there, each sub-directory will have a grbl directory inside of which is a grbl.ino file. Simply open that up with the Arduino IDE and you can work as normal.
Unlike with a normal Grbl build process there is absolutely no need to install any of this as a library. Just load as above, build, and upload to your board. That's it!