Describe simple renderers that could use HDR cubemap textures and SH lighting

[kearwood]

As per the feedback by @elalish in the initial PR:

When an HDR Cube Map texture is available, shadows only have to consider occlusion of other rendered objects in the scene.
@elalish: Not quite sure what this sentence is saying... I'm pretty sure this is hard.

We should add some details explaining how a simpler, non-PBR, renderer (eg, a standalone GLSL shader) could give acceptable results:

Doing this 100% correctly is very hard; however, there are many simple approximations that are commonly used. In particular, it may be sufficient in some cases to combine a baked ambient occlusion map with a IBR shader that needs no realtime dynamic lights.

A simple, non-physically based implementation may simply index the HDR cube map using a surface normal, and blend it with an albedo term representing the color of the surface using operators representing the artists intent.