How to integrate a new version control plugin into Unity?

Hi,

I'm exploring the VersionControlPlugins repo on BitBucket and am trying to understand how to integrate a new version control plugin into the Unity editor. The readme there doesn't mention anything about registering the plugin in Unity.

How do you add a new plugin to the dropdown of Version Control selection in Edit->Project Settings->Editor? I see that UnityEditor.VersionControl.Plugin.availablePlugins lists them, but I don't see a mechanism for adding.

Am I missing some existing documentation?

Thanks!