Add simple PR comment sub-action (#636)

* Add simple PR comment sub-action

* Update rolldown config

* Add changeset

* Update readme

* Use TLA

* Add prefix

* fix merge

* Update

pr-comment/README.md

@@ -0,0 +1,52 @@
+# @changesets/action/pr-comment
+
+A simple GitHub Action to comment on PRs aimed to complement [`@changesets/action/pr-status`](../pr-status/README.md).
+
+This action is intentionally simple without advanced features. Check out other actions if so, such as [mshick/add-pr-comment](https://github.com/marketplace/actions/add-pr-comment) and [peter-evans/create-or-update-comment](https://github.com/marketplace/actions/create-or-update-comment).
+
+See the [action metadata](action.yml) for details on the inputs and outputs.
+
+## Example setup
+
+```yaml
+name: PR Comment
+
+on:
+  pull_request:
+
+jobs:
+  pr-comment:
+    runs-on: ubuntu-slim
+    permissions:
+      pull-requests: write # to create and update comments on PRs
+    steps:
+      - uses: changesets/action/pr-comment@v1
+        with:
+          body: Hello world!
+```
+
+When called repeatedly, the action will update the comment it created by default. If you use this action to create different types of comments, pass an `update-id` value to differentiate them.
+
+```yaml
+jobs:
+  pr-comment:
+    # ...
+    steps:
+      - uses: changesets/action/pr-comment@v1
+        with:
+          body: Hello world!
+          update-id: my-tag
+```
+
+If you want to always create new comments, pass an empty value to `update-id`.
+
+```yaml
+jobs:
+  pr-comment:
+    # ...
+    steps:
+      - uses: changesets/action/pr-comment@v1
+        with:
+          body: Hello world!
+          update-id: ""
+```