Docs: Clarify what the "window" feature is

[parker-research]

The README explains that the "window" feature must be enabled to use the window module.

Can you clarify what the "window" feature is? It doesn't seem to be a Rust/cargo feature on the three-d module; is it on the other winit/egui/etc. rendered modules? Or is it a legacy carry-over that just needs a tiny update?

[P-GLEZ]

The Cargo.toml file shows it's a cargo feature; what's missing from the documentation is the fact that is a default feature

[parker-research]

Makes sense - I think I was deficient in my initial investigation, thanks!