Readme update

[Firestar99]

Requires #132 (comment)

• mention the new templates: https://github.com/Rust-GPU/rust-gpu-template
• explain the crate dependency path
  • via cargo-gpu-install from Split install action into the new crate cargo-gpu-install #132
• The first commit is the final state we want to get to, the other commits "undo" some of the final state to get to our current state. As more and more things are built, we can revert these "undo" commits.
• feel free to discuss!