Improve documentation of lighting for shader generation

[kwokcb]

Proposal

There is little documentation on how to set up lighting, and how to set appropriate generation options. Proposal is to address this and perhaps provide some basic examples. One issue that has come up is performance vs fidelity.

For example: this doc could be enhanced.

[pmady]

Hi I am first time contributor for this repo, can help in improving the docs. Also open for discussions. Thanks