In general, you should install a release zip, rather than clone this repo. If you do want to use the git version directly, you need to clone into your addons folder and recursively init/update all submodules.
Quick and dirty:
cd ${WARCRAFT_DIR}/Interface/AddOns/
git clone https://github.com/Itarater/XRP.git
cd XRP/
git submodule update --init --recursiveCopyright and license notices are included in the headers of all substantial files. Files without copyright or license notices are considered by the author to not meet the theshold of originality required for copyright, thus consisting solely of non-copyrightable material, and may be treated as such.