Document dependency requirements

[heaths]

Document the dependency requirements for what gets defined in the workspace Cargo.toml, and what crates' Cargo.tomls 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 https://azure.github.io/azure-sdk/rust_implementation.html.

[hallipr]

When publishing a package, I would expect the workspace dependency to already have the current, to-be-deployed version and a path:

[workspace.dependencies.azure_core]
version = "0.25.0"
path = "sdk/core/azure_core"

Looking at this dependency, any dependent package build would infer that it depends on unreleased 0.25.0 and that it should build against the package at path sdk/core/azure_core.

If we change our post-release PR to bump versions and remove path dependencies, we will leave dependent packages in a releasable, stable state while still allowing forward dev on the dependency package:

After release of azure_core@0.25.0:

sdk/core/azure_core/Cargo.toml

[package]
name = "azure_core"
-version = "0.25.0"
+version = "0.26.0"    <-- version bump

/Cargo.toml

[workspace.dependencies.azure_core]
version = "0.25.0"
- path = "sdk/core/azure_core"  

Now, this dependency looks like any other workspace dependency for a released package. Even better, there's no conflict between the dependency version of azure_core (0.25.0) and the version being handled by -Zpackage-workspace (0.26.0)

[heaths]

And then anyone - at least post-GA - who needs in-dev versions of azure_core or other core crates would take a path dependency and we'd do the normal release cycle: release a new azure_core, update the workspace, update the crates to use the now-updated workspace dependency on azure_core, and all is well.

But that's post-GA. Right now, everything is in flux.