[Markdown] Contributing: Add section about release branch PRs

In an effort to centralize the release branch PR form and release
process info in general, dedicate a section to this for repositories and
forum announcements to refer to instead of each publishing their own
copies.

Author: AnthonyLatsis

contributing/_contributing-code.md

@@ -111,6 +111,35 @@ For C or C++ source or header files, the code header should look this:
 
 The divider lines should be exactly 80 characters wide to aid in adherence to the code style guidelines.  The bottom section contains an optional description intended for generated documentation (these lines begin with `///` rather than `//`).  If there is no description, this area can be skipped.
 
+### Release Branch Pull Requests
+
+A pull request targeting a release branch (`release/x.y`) cannot be merged
+without an approval by the corresponding release manager.
+In order for a change to be considered for inclusion in a release branch, the
+pull request must have:
+
+* A title that starts with a `[x.y]` tag matching the release version of the
+  target branch
+
+* The following form filled out in its description:
+
+  ```
+  - **Explanation**: A description of the change. This can be brief, but it
+    should be clear.
+  - **Scope**: An assessment of the impact and importance of the change. For
+    example, is the change a source-breaking language change?
+  - **Issues**: References to issues the change resolves, if any.
+  - **Original PRs**: Links to the mainline branch pull requests, the changes
+    from which make up this pull request, if any.
+  - **Risk**: The (specific) risk to the release for taking this change.
+  - **Testing**: The specific testing that has been done or needs to be done to
+    further validate any impact of this change.
+  - **Reviewers**: The code owners that approved the mainline branch pull
+    requests. If any part of the mainline branch changes was not approved by a
+    respective code owner, provide a reason. Technical review can be delegated
+    by a code owner or otherwise requested as deemed appropriate or useful.
+  ```
+
 ### Code Review
 
 The Swift project relies heavily on code review to improve software quality: