We still haven't written the documentation, which is a bit complex, and we are still completing a PaintKit too. Those will all come once they are done, but we wanted to give 3rd parties some head start so they could start working on add-ons as soon as possible.
I suggest joining the GSX Creators channel on Discord, there's a specific sub-channel there which discusses external packages. I'm not the admin there, since it's not an FSDT-own server so, after you join, send a message to the admins and ask access to that section.
