From http://doc.crates.io/manifest.html:

# This points to a file in the repository (relative to this `Cargo.toml`). The
# contents of this file are stored and indexed in the registry.
readme = "..."

The problem is that these kinds of links cannot point to outside of package root, since anything outside of package root won't be able to be packaged in the first place.

With workspaces and not all Cargo manifest files being on the (git) repository top-level anymore, usage of word repository here is now confusing and should be avoided in package-level documentation.

The notion of package root (vs. repository) is also important important in explaining workspace.members and workspace.exclude, as I've noted here: #4268 (comment)