stacking recommendation banner

Author: skarim

docs/src/assets/screenshots/stack-recommendation-banner.png

@@ -25,11 +25,13 @@ gh stack submit
 
 You can also create stacks entirely from the GitHub UI — create the first PR normally, then when creating subsequent PRs, select the option to add them to a stack. See [Creating a Stack from the UI](/gh-stack/guides/ui/#creating-a-stack-from-the-ui) for a walkthrough.
 
+If you already have open PRs whose branches line up, GitHub will detect and suggest turning them into a stack. See [Turning Existing PRs into a Stack](/gh-stack/guides/ui/#turning-existing-prs-into-a-stack).
+
 ### How do I add PRs to my stack?
 
 Use `gh stack add <branch-name>` to add a new branch on top of the current stack. When you run `gh stack submit`, a PR is created for each branch, and they are linked together as a Stack on GitHub.
 
-You can also add PRs to an existing stack from the GitHub UI. See [Adding to an Existing Stack](/gh-stack/guides/ui/#adding-to-an-existing-stack) for details.
+You can also add PRs to an existing stack from the GitHub UI — either a brand-new PR or an already-open PR (via the recommendation banner), added to the top of the stack. See [Adding to an Existing Stack](/gh-stack/guides/ui/#adding-to-an-existing-stack) for details.
 
 ### How can I modify my stack?
 

docs/src/assets/screenshots/stack-recommendation-dialog-add.png

@@ -43,9 +43,31 @@ After creating the PR, you'll see the stack map appear in the header, showing bo
 
 Repeat this process for each additional PR in the stack — each one targets the branch of the PR before it.
 
+## Turning Existing PRs into a Stack
+
+If you already have open PRs whose branches line up (each PR's base branch is the head branch of the PR below it), GitHub recognizes the chain and shows a **recommendation banner** offering to turn them into a stack.
+
+![Banner recommending that eligible PRs be turned into a stack](../../../assets/screenshots/stack-recommendation-banner.png)
+
+Click the banner to open a dialog that previews the stack, listing each PR in order from top to bottom. Review it and confirm to link the PRs together into a stack.
+
+![Dialog previewing the stack before it's created](../../../assets/screenshots/stack-recommendation-dialog-create.png)
+
+Once you confirm, the PRs are stacked and the stack map appears in each PR's header.
+
 ## Adding to an Existing Stack
 
-If a stack already exists and you want to add a new PR to it:
+You can add a PR to the top of an existing stack either when you create the PR or after it already exists.
+
+### Add an existing PR
+
+If you already have an open PR whose base branch is the head branch of the stack's topmost PR, GitHub shows a **recommendation banner** on that PR, giving you an option to add it to the stack. Click it to preview and confirm, and the PR is added to the top of the existing stack.
+
+![Recommendation dialog for adding an existing PR to a stack](../../../assets/screenshots/stack-recommendation-dialog-add.png)
+
+### Create a new PR on the stack
+
+To create a brand-new PR directly on top of the stack:
 
 1. Open a PR in the stack, click the stack icon in the header, and click **Add**.