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

.github/PULL_REQUEST_TEMPLATE/release.md

@@ -0,0 +1,46 @@
+<!--
+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 change. 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?
+  -->
+* 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?
+  -->
+* Reviewers:
+  <!--
+  The code owners that reviewed the original PR. One or more code owners of
+  the impacted components should review the change. Technical review can be
+  delegated by a code owner or otherwise requested as deemed appropriate or
+  useful.
+  -->
+