Like most open source projects the zyn team loves to see interested developers help out. If you're interested in helping out coding we recommend checking out the issue trackers for bugs, features, and patches. There you should be able to find some issues marked 'beginner'. These issues should involve relatively few different source files and ideally it should be possible to finish them in a single sitting. Resulting changes should be contributed back through either the patch tracker or via a git pull request.
People are always looking for new instruments to experiment with. For a one-off instrument musical artifacts hosts individual instruments for a variety of open source synths and effects. For larger collections of patches they can be shared via the mailing list.
Open source documentation doesn't always cover the topics you're looking for quite in enough depth or breath and zyn's is no exception. Features are constantly getting added and the interface is currently undergoing a major overhaul. As such there are plenty of gaps which need filling. Currently most of the documentation has been migrated to an asciidoc based version stored in the /doc folder of the git version. If you're interested in helping this information become more complete please just add some information to these files and submit the changes either via the patch tracker or via a pull request.
Any donations are appreciated and they're currently accepted by two developers. The project was started in March 2002 by Nasca Octavian Paul. He began to write ZynAddSubFX in order to create a synthesizer which could produce beautiful sounds, while being freely available to anybody who needs it. Since 2009, ZynAddSubFX has been lead by Mark McCurry. In these few years some major architectural changes have been brought about which will culminate in the 3.0.0 UI rewrite release.