Change to link to "Contributing to Rust" chapter of `rustc` Dev Guide, primarily on the basis that the GitHub "first contribution" Issue "pop-up" says "Be sure to review the [contributing guidelines] and [code of conduct]" and links to this file.

When/if the guide/"Getting Started" section gets revised to not be `rustc`-specific the linked section can be changed.

In the meantime this prevents leading first time contributors into a confusing cul de sac.