Set up /backport action command

.github/workflows/backport.yaml

@@ -0,0 +1,21 @@
+name: Backport PR to branch
+on:
+  issue_comment:
+    types: [created]
+
+permissions:
+  contents: write
+  issues: write
+  pull-requests: write
+  actions: write
+
+jobs:
+  backport:
+    uses: dotnet/arcade/.github/workflows/backport-base.yml@main
+    with:
+        pr_description_template: |
+          Backport of #%source_pr_number% to %target_branch%
+
+          Requested in: %source_pr_url%#issuecomment-%trigger_comment_id%
+
+          /cc %cc_users%

CONTRIBUTING.md

@@ -8,6 +8,22 @@ When you submit a pull request, a CLA-bot will automatically determine whether y
 
 If you want to submit a feature or a substantial code contribution, please discuss it first with the the team by [filing an issue](https://github.com/dotnet/project-system/issues/new), making sure it follows our [Roadmap](docs/repo/roadmap.md).
 
+## Backporting Changes
+
+To backport a merged pull request to a release branch, add a comment to the PR with the following format:
+
+```
+/backport to <branch-name>
+```
+
+For example, to backport to the `dev17.13.x` branch:
+
+```
+/backport to dev17.13.x
+```
+
+The backport action will automatically create a new PR targeting the specified branch. This command is only available to users with Write or Admin permissions on the repository. Available release branches include `dev17.13.x`, `dev17.9.x`, `dev17.8.x`, `dev17.7.x`, `dev17.4.x`, and `dev16.11.x`.
+
 For additional information, see the following:
 
 - [Getting Started](docs/repo/getting-started.md)