Document dependency requirements

Document the dependency requirements for what gets defined in the workspace `Cargo.toml`, and what crates' `Cargo.toml`s can do e.g., `dev-dependencies` to other projects in the repo should always be `path` dependencies, or inherited from the workspace if those are defined as `path` dependencies, as we do for `azure_core_test`.

I can think of two places we could put this, with my preference on the first because it's right in the repo and more likely to be seen.

1. Root `CONTRIBUTING.md` - either a short block of text or, if longer with examples, a link to another doc under the `doc/` folder e.g., `doc/dependencies.md`.
2. As part of the guidelines - probably in .