Pull requests are always welcome.
If you found a bug and wrote a fix for it, just create a pull request.
For smaller enhancements, just create a pull request.
If you plan to implement any bigger new features, please create an new issue first so we can discuss if and how this feature can be implemented. If you want to work on implementing a feature from an existing issue, please leave a note there to avoid duplicate work.
By submitting any code, you agree to make that code available under the GNU General Public License v3.0 (and any later version), and that you have the necessary rights to do so. You cannot revoke this later. However, your copyright on your own code remains unaffected by this, you can still license your own code to other parties (under the same or a different license).