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

[David Spickett via llvm-commits]

================
@@ -29,8 +31,65 @@ 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
+=====================
+
+To separate related changes or to break down a larger PR into smaller, reviewable
+pieces, use "stacked pull requests" — this helps make the review process
+smoother. While GitHub does not natively support stacked pull requests, there
+are several common alternatives.
+
+To illustrate, assume that you are working on two branches in your fork of the
+``llvm/llvm-project`` repository, and you want to eventually merge both into
+``main``:
+
+- `feature_1`, which contains commit `feature_commit_1`
+- `feature_2`, which contains commit `feature_commit_2` and depends on
+  `feature_1` (so it also includes `feature_commit_1`)
+
+Your options are as follows:
+
+#. Two PRs with a dependency note
+
+   Create PR_1 for `feature_1` and PR_2 for `feature_2`. In PR_2, include a
+   note in the PR summary indicating that it depends on PR_1 (e.g.,
+   “Depends on #PR_1”).
+
+   To make review easier, clearly highlight which commits are part of the base
+   PR and which are new. This helps reviewers focus only on the incremental
----------------
DavidSpickett wrote:

Not immediately obvious how to "clearly highlight", but it's pretty simple once you know: "The first N commits are previous PR" or "the last N commits are new"

Worth including that suggestion to avoid another point of uncertainty for contributors.

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