add callout in readme

Added note about private preview status and waitlist sign up link to top of Readme

Author: skarim

README.md

@@ -4,6 +4,10 @@ A GitHub CLI extension for managing stacked branches and pull requests.
 
 Stacked PRs break large changes into a chain of small, reviewable pull requests that build on each other. `gh stack` automates the tedious parts — creating branches, keeping them rebased, setting correct PR base branches, and navigating between layers.
 
+> [!NOTE]
+> Stacked PRs is currently in private preview. This CLI and the referenced functionality will not work unless the feature has been enabled for your repository.
+> You can sign up for the waitlist at [gh.io/stacksbeta](https://gh.io/stacksbeta).
+
 ## Installation
 
 ```sh