Add documentation about third-party registries.

Author: ehuss

src/doc/src/reference/registries.md

@@ -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