.github: Add a pull request template for release branch PRs

.github/PULL_REQUEST_TEMPLATE/release.md

@@ -0,0 +1,25 @@
+<!--
+In order for a pull request to be considered for inclusion in a release branch
+(release/x.y) after it has been cut, the pull request must have the following
+information filled out in its description.
+
+To draft a pull request using this template, append '&template=release.md' to
+a pull request preview URL (the "Open a pull request" page), or use the
+following URL template:
+
+https://github.com/apple/swift/compare/main...<username>:swift:<my-branch>?expand=1&template=release.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.
+* Issues: References to issues the change resolves, if any.
+* Original PR: Link to the main branch version of this pull request.
+* 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.
+