Not every available package is stored on OpenWrt Yún repository: as there are thousands of them, separating the core ones from the rest allows to keep things cleaner.
When you build the image on your computer, file feeds.conf.default is used to download additional packages list. As you can see, the first line mentions this very repository.
Just as the main repository, this one is a fork of the official OpenWrt one, with a handful of Yún specific additions and customizations.
If you wish to contribute to the list of available packages, please consider forking this repo. Change the url on feeds.conf.default to point to your fork, play with it and once you ready, share your work with the community by opening a pull request