The primary documentation is the PDF included inside the Asset Store Package which you downloaded when you bought/upgraded/installed the asset – this is always up to date. You can see/download a recent version below (PDF Manual).
The plugin also contains many demo scenes, in the “Demo Scenes” subfolder – some of these are described in the PDF in more detail – but feel free to play with them, use/modify the scripts etc.
Complete API docs for customizing/extending almost any aspect of SnP:
- API v2.2 – November 2018
- NB: contains one class that’s currently only available to v2.3/v3.0 alpha-testers – DragNDropAction3D
Online (written) tutorials
There are also some old online tutorials (2016) here (but you’re better off using the PDF manual described above):