[llvm] [docs][GitHub] Document alternative approach to stacked PRs (PR #132424)

[Aiden Grossman via llvm-commits]

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@@ -29,8 +29,23 @@ intended to be able to support "stacked" pull-request. Do not create any branche
 llvm/llvm-project repository otherwise, please use a fork (see below). User branches that
 aren't associated with a pull-request **will be deleted**.
 
+Stacked Pull Requests
+=====================
+
+GitHub does not natively support stacked pull requests. There are two common
+alternatives:
+
+  * Add a note in your PR summary indicating that your patch is part of a
+    series or depends on another PR (e.g., “Depends on #123456”). It also helps
+    to highlight which commits belong to other PRs, so reviewers can focus only
+    on the relevant changes.
+  * Use Graphite (described below), a tool that supports stacked PR workflows.
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boomanaiden154 wrote:

SPR (https://github.com/spacedentist/spr) also works and I think is documented somewhere in this file. It is used reasonably commonly by llvm developers.

https://github.com/llvm/llvm-project/pull/132424