docs(README): point users to the successor pnpm/setup action (#282)

* docs(README): point users to the successor pnpm/setup action

The pnpm setup action has moved to `pnpm/setup`, which installs
pnpm v11+ as a self-contained native executable and can install a
JavaScript runtime (Node.js, Bun, or Deno) in the same step,
replacing `actions/setup-node`.

Add a notice at the top of the README, a migration section with a
before/after workflow example and an input/output mapping table,
and mention the successor in the Notes section. The rest of the
docs stay intact since `pnpm/action-setup` remains the way to
install pnpm v10 and older.

Signed-off-by: Kevin Cui <bh@bugs.cc>

* docs(README): address review feedback on migration section

Add an explicit version: 11 to the migration example and note when the
input can be omitted, since pnpm/setup requires pnpm v11+ and a repo
migrating from version: 10 may have no packageManager field or one that
pins v10.

Move the v2 upgrade warning below the page title so the two callouts
are no longer adjacent blockquotes (markdownlint MD028), keeping the
successor notice and the legacy warning as separate blocks.

Fix the comma splice and use "set up" as the verb in the Notes
section.

Signed-off-by: Kevin Cui <bh@bugs.cc>

---------

Signed-off-by: Kevin Cui <bh@bugs.cc>

README.md

@@ -1,11 +1,61 @@
-> ## :warning: Upgrade from v2!
+> [!IMPORTANT]
+> **This action has a successor: [`pnpm/setup`](https://github.com/pnpm/setup).**
 >
-> The v2 version of this action [has stopped working](https://github.com/pnpm/action-setup/issues/135) with newer Node.js versions. Please, upgrade to the latest version to fix any issues.
+> For pnpm v11 and newer, use [`pnpm/setup`](https://github.com/pnpm/setup) instead. It downloads pnpm's self-contained release binary (no Node.js or npm required) and can install a JavaScript runtime (Node.js, Bun, or Deno) in the same step, replacing `actions/setup-node`.
+>
+> `pnpm/action-setup` remains the action to use for installing pnpm v10 and older. See [Migrating to pnpm/setup](#migrating-to-pnpmsetup) below.
 
 # Setup pnpm
 
 Install pnpm package manager.
 
+> ## :warning: Upgrade from v2!
+>
+> The v2 version of this action [has stopped working](https://github.com/pnpm/action-setup/issues/135) with newer Node.js versions. Please, upgrade to the latest version to fix any issues.
+
+## Migrating to pnpm/setup
+
+[`pnpm/setup`](https://github.com/pnpm/setup) installs pnpm v11+ as a native standalone executable and can install Node.js, Bun, or Deno in the same step, so a typical workflow no longer needs `actions/setup-node` or an explicit `pnpm install` step:
+
+```yaml
+steps:
+  - uses: actions/checkout@v6
+
+  # Before:
+  # - uses: pnpm/action-setup@v6
+  #   with:
+  #     version: 10
+  #     cache: true
+  # - uses: actions/setup-node@v4
+  #   with:
+  #     node-version: 22
+  # - run: pnpm install
+
+  # After:
+  - uses: pnpm/setup@v1
+    with:
+      version: 11
+      runtime: node@22
+      cache: true
+```
+
+The `version` input can be omitted only when `packageManager` (or `devEngines.packageManager`) in `package.json` declares pnpm v11 or newer; otherwise keep it explicit, since `pnpm/setup` requires pnpm v11+.
+
+Input and output changes:
+
+| `pnpm/action-setup` | `pnpm/setup` | Notes |
+| ------------------- | ------------ | ----- |
+| `version` | `version` | Must resolve to pnpm v11 or newer. As before, it can be omitted when `packageManager` (or `devEngines.packageManager`) is set in `package.json`. |
+| `dest` | `dest` | Unchanged. |
+| `run_install` | `install` | `pnpm/setup` runs `pnpm install` automatically when a `package.json` is present (`install: true` by default); set `install: false` to skip it. The object/array form (`recursive`, `cwd`, `args`) is not supported — run those commands in separate steps. |
+| `cache` | `cache` | Unchanged. |
+| `cache_dependency_path` | `cache-dependency-path` | Renamed to kebab-case. |
+| `package_json_file` | `package-json-file` | Renamed to kebab-case. |
+| `standalone` | removed | `pnpm/setup` always installs the standalone native executable. |
+| n/a | `runtime` | New: installs Node.js, Bun, or Deno (e.g. `node@22`, `bun@latest`, `deno@2`), or reads `devEngines.runtime` from `package.json`. |
+| n/a | `token` | New: GitHub token for release lookup; defaults to `${{ github.token }}` and rarely needs to be set. |
+| `bin_dest` (output) | `bin-dest` (output) | Renamed to kebab-case. New outputs `runtime-name` and `runtime-version` describe the installed runtime. |
+
 ## Inputs
 
 ### `version`
@@ -187,7 +237,7 @@ jobs:
 
 ## Notes
 
-This action does not setup Node.js for you, use [actions/setup-node](https://github.com/actions/setup-node) yourself.
+This action does not set up Node.js. Use [actions/setup-node](https://github.com/actions/setup-node) yourself. If you are on pnpm v11 or newer, [`pnpm/setup`](https://github.com/pnpm/setup) can install pnpm and Node.js in a single step.
 
 ## License