Nice to hear that people are developing plugins!
I assume you found these resources?
https://github.com/Ultimaker/Uranium/wiki/Creating-plugins
https://github.com/Ultimaker/Uranium/wiki/Backend-&-Plugin-structure
https://github.com/Ultimaker/Uranium/wiki/Plugin-Types
I just noticed not all of them are fully up to date, but they cover most of what you need to know.
With regards to running it from source, you're best of running it from the 15.06 branch (as that is the release branch for 15.06, it only has bug fixes). On 1st of september we will go into feature freeze for 15.10 and release the open beta for that version. We did quite a few changes for 15.10 (re-worked the output device code for instance!), so you might want to wait a bit for that.
If you have any specific questions, don't hesitate to ask. If you tag me in a post It's pretty much guaranteed that I read it.