Alternative way to install the repository, using a link.
Add https://doko-desuka.github.io to your Kodi file sources by going to Settings > File Manager > add a new source for that link, then go to Add-ons > Add-ons browser (the open box icon on the top-left) > Install from zip file > choose the name you gave to that new source.
This alternative method is used because this repository almost never gets updates, so it'd make no sense to have your Kodi constantly polling for changes every time it starts up.