We welcome pull requests from anyone. The master branch is the primary branch for development, and if you wish to add new functionality, it probably belongs there. We attempt to maintain recent previous branches and welcome bug fixes and backports for those. Currently, the actively maintained branches are sumo, rocko, and pyro. Previously, morty was also a stable branch, but it has not been updated or actively maintained for a while.
If you are developing with meta-updater, it may be helpful to read the README and other documentation for this repo, aktualizr, and the updater-repo, particularly the sections about development and debugging.