You definitely don't need to fork the repo! Although, I think it is easier to develop plugins by running Kibana from the sources (rather than from a production build), as you'll get more dev-friendly stack traces if anything goes wrong.
This is probably the best documentation we've got at the moment:
What I have done is just poked around in the Kibana codebase to see how Kibana's built-in plugins are written, and I've gone on from there.
But as I said, it depends on your usecase as to what you should build.