The addon SDK documentation is now hosted on construct.net at https://www.construct.net/make-games/manuals/addon-sdk. The old provisional documentation won't be updated any more, please refer to the new documentation only.
Today's r48 update also adds support for effect addons and dependencies. We've done some work to revise the effects engine to be more robust in C3, so note for effect addons some of the uniforms are different to what's in C2. The documentation covers all the uniforms available.