Add link in previous call for testing.

Author: ehuss

posts/2022-06-22-sparse-registry-testing.md

@@ -5,6 +5,9 @@ author: Arlo Siemsen
 team: The Cargo Team <https://www.rust-lang.org/governance/teams/dev-tools#cargo>
 ---
 
+> Note: Sparse registry support has been stabilized for the 1.68 release.
+> See [Help test Cargo's new index protocol](../../../inside-rust/2023/01/30/cargo-sparse-protocol.html) for updated information.
+
 The Cargo nightly [`sparse-registry`][sparse-registry] feature is ready for testing. The
 feature causes Cargo to access the crates.io index over HTTP, rather than git. It can
 provide a significant performance improvement, especially if the local copy of