docs(unstable): Link out to the Plumbing commands effort

This is intended to help people discover where to go to discuss plumbing
command efforts.

Author: epage

src/doc/src/reference/unstable.md

@@ -129,6 +129,7 @@ Each new feature described below should explain how to use it.
     * [warnings](#warnings) --- controls warning behavior; options for allowing or denying warnings.
     * [Package message format](#package-message-format) --- Message format for `cargo package`.
     * [`fix-edition`](#fix-edition) --- A permanently unstable edition migration helper.
+    * [Plumbing subcommands](https://github.com/crate-ci/cargo-plumbing) --- Low, level commands that act as APIs for Cargo, like `cargo metadata`
 
 ## allow-features