Mention somewhere that the latest commit should be used #479

In the initial crate setup as mentioned in the book, it tells you to setup spirv_builder by adding the 0.9 version to your Cargo.toml: https://rust-gpu.github.io/rust-gpu/book/writing-shader-crates.html#using-spirv-builder

This of course does not work because the last spirv_builder release is ancient. In fact, the rust-toochain.toml mentioned immediately before this does not match the one from spirv_builder so upon doing this the crate won't even compile. This is easy enough to fix:

[build-dependencies]
# maybe use a commit if you need some semblance of stability
spirv-builder = { git = "https://github.com/Rust-GPU/rust-gpu", revision = "master" }

Figuring this out however was challenging. I feel like this should be mentioned somewhere in this section of the book.