Add "Opening a PR for Release Branch" section to contributing document

.github/PULL_REQUEST_TEMPLATE/release_branch.md

@@ -0,0 +1,7 @@
+* **Explanation**: <!-- A description of the issue being fixed or enhancement being made. This can be brief, but it should be clear. -->
+* **Scope**: <!-- An assessment of the impact/importance of the change. For example, is the change a source-breaking language change, etc. -->
+* **Issue**: <!-- The GitHub Issue link if the change fixes/implements an issue/enhancement -->
+* **Original PR**: <!-- Pull Request link from main branch -->
+* **Risk**: <!-- What is the (specific) risk to the release for taking this change? -->
+* **Testing**: <!-- What specific testing has been done or needs to be done to further validate any impact of this change? -->
+* **Reviewer**: <!-- One or more code owners for the impacted components should review the change. Technical review can be delegated by a code owner or otherwise requested as deemed appropriate or useful. -->

CONTRIBUTING.md

@@ -79,6 +79,34 @@ For more information and instructions, read the GitHub docs on [forking a repo](
 
 Once you've pushed your branch, you should see an option on this repository's page to create a PR from a branch in your fork.
 
+## Opening a PR for Release Branch
+
+In order for a pull request to be considered for inclusion in a release branch (e.g. `release/6.0`) after it has been cut, it must meet the following requirements:
+
+1. The title of the PR should start with the tag `[{swift version number}]`. For example, `[6.0]` for the Swift 6.0 release branch.
+
+1. The PR description must include the following information:
+
+    ```md
+    * **Explanation**: A description of the issue being fixed or enhancement being made. This can be brief, but it should be clear.
+    * **Scope**: An assessment of the impact/importance of the change. For example, is the change a source-breaking language change, etc.
+    * **Issue**: The GitHub Issue link if the change fixes/implements an issue/enhancement.
+    * **Original PR**: Pull Request link from the `main` branch.
+    * **Risk**: What is the (specific) risk to the release for taking this change?
+    * **Testing**: What specific testing has been done or needs to be done to further validate any impact of this change?
+    * **Reviewer**: One or more code owners for the impacted components should review the change. Technical review can be delegated by a code owner or otherwise requested as deemed appropriate or useful.
+    ```
+
+> [!TIP]
+> The PR description can be generated using the [release_branch.md](https://github.com/apple/swift-syntax/blob/main/.github/PULL_REQUEST_TEMPLATE/release_branch.md) [pull request template](https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/about-issue-and-pull-request-templates). To use this template when creating a PR, you need to add the query parameter:
+> ```
+> ?expand=1&template=release_branch.md
+> ```
+> to the PR URL, as described in the [GitHub documentation on using query parameters to create a pull request](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/using-query-parameters-to-create-a-pull-request). 
+> This is necessary because GitHub does not currently provide a UI to choose a PR template.
+
+All changes going into a release branch must go through pull requests that are approved and merged by the corresponding release manager.
+
 ## Review and CI Testing
 
 After you opened your PR, a maintainer will review it and test your changes in CI (*Continuous Integration*) by adding a `@swift-ci Please test` comment on the pull request. Once your PR is approved and CI has passed, the maintainer will merge your pull request.