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

[Simon Tatham via llvm-commits]

================
@@ -29,8 +31,48 @@ 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. However, there are
+several common alternatives.
+
+To illustrate, let's assume you're 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 dependency note
+
+  Create PR_1 for branch feature_1 and PR_2 for branch feature_2. In PR_2, add a
+  note in the PR summary indicating that it is part of a series or depends on
+  another PR (e.g., “Depends on #PR_1”). It's also helpful to highlight which
+  commits belong to the base PR, so reviewers can focus on the new changes.
+
+#. User branches in `llvm/llvm-project`
+
+  Create user branches in the main repository, as described
+  :ref:`above<github_branches>`. Then:
+
+  * Create a pull request from `users/<username>/feature_1` → `main`
+  * Create another from `users/<username>/feature_2` →
+    `users/<username>/feature_1` Once `feature_1` is merged, you can update the
+    base of the second PR to target main.
+
+#. Use a stacked PR tool
+
+  Use a tool like SPR or Graphite (described below) to manage stacked PRs more
+  easily.
+
+Each of these approaches can help streamline the review process. Choose the one
----------------
statham-arm wrote:

I don't have a strong opinion _where_ we explain the reasoning for "squash and merge", but I think it would be good to have it explained _somewhere_, if only so that people used to the normal way of doing things don't assume the answer is "because we hate you and want you to have to do everything the most complicated way possible" :slightly_smiling_face:

(After all, the fact that we have to list several different workarounds here, including two that depend on an extra tool, shows that there clearly _is_ a downside to "squash and merge", so it's reasonable for people to want to know what benefit LLVM thinks is worth all that inconvenience!)

But if you personally don't know the answer (and I can't blame you, because I don't either) then fair enough!

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