Add user README inside plugin folder

[msanatan]

If your asset contains any code (scripts, shaders) - we ask that you include offline documentation in the format of pdf or rtf with your submission, as it is mandatory for all packages that include scripts or other components that require set up. Your documentation must be organized with a table of contents and numbered, written in English and have no grammar mistakes. Create a setup guide with a step-by-step tutorial (pdf or video), as well as a script reference if users will need to do any coding. If your asset contains art (3D models, sprites) and you used code to set up a demo scene, you may skip this step.

From asset validation

[msanatan]

Closed by #225 and #230