[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
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boomanaiden154 wrote:

With user branches we sort of have stacked pull requests. The supper still isn't great though.

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