Contributes are always welcome, no matter if it's additional documentation, a bugfix, a new feature to an existing extension or complete new extension that you are adding to the repository.
If you are looking for an easy first contribution have a look at this list.
Currently one of the biggest tasks we are working on is migrating some of the extensions from the mbeddr platform to this repository. If you like to help have a look at this page.
Should you not feel comfortable to start with a code contribution additions to our documentation are always very welcome. Our documentation is in this repository as well. You can have edit it in your browser right a way if you like.
For bugfixes, documentation and small new features you can open a pull request right away. Bigger features or new extensions should get some discussion in an issue just to make sure we are all on the same page about what is going to be done. If you are unsure what to do don't hesitate to open a issue and ask for help.