Community contributions are heartily welcome in the Turris project. We value every contribution and every contributor. As Turris OS is quite complex, here are a few hints on how and where to contribute.
Turris OS spans over multiple repositories. The main one is turris-os - a fork of the OpenWRT base repo with some modifications by us and some cherry-picked modifications from LEDE. Upstream changes in LEDE are merged into it gradually, but it sometimes takes quite some time for the changes to get into it from upstream. Whenever you want to contribute to this repository, also try to simultaneously contribute to LEDE. This way your contribution won't get lost during the next rebase.
We also use various upstream OpenWRT feeds like packages. If you want something changed there, send your contribution to upstream and we will pick it up on next update. We tend to update feeds quite often. If you would like a new package included in Turris OS, please send it to packages.
The last repo is our own packages repo turris-os-packages, which we maintain ourselves. This contains packages that are either Turris specific or diverged too much from upstream and it therefore makes more sense to maintain them locally.
In our repositories all development happens in the test branch. So if your contribution should go into one of our repositories, send a pull request against the test branch. You can use the usual GitHub fork/pull-request workflow.