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

[Andrzej Warzyński 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
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banach-space wrote:

> Is it just that people don't know about spr? 

Yup, I think that we need to do a bit of "advertising". So, once this is approved, I will post something on Discourse to draw people's attention to the available options.

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