message: Add documentation about third-party registries.
diff: @@ -100,9 +100,9 @@ A full-featured registry that supports publishing will additionally need to
  have a web API service that conforms to the API used by Cargo. The web API is
  documented below.
  -At this time, there is no widely used software for running a custom registry.
  -There is interest in documenting projects that implement registry support, or
  -existing package caches that add support for Cargo.
  +Commercial and community projects are available for building and running a
  +registry. See <https://github.com/rust-lang/cargo/wiki/Third-party-registries>
  +for a list of what is available.
  ### Index Format
  