[Docs] Add "Updating CI Jobs" section to CONTRIBUTING.md (jpsim#284)

Author: jpsim

CONTRIBUTING.md

@@ -14,3 +14,10 @@ We follow the same syntax as [CocoaPods' `CHANGELOG.md`](https://github.com/Coco
 4. A list of Markdown hyperlinks to the issues the change addresses. One entry
    per line. Usually just one.
 5. All `CHANGELOG.md` content is hard-wrapped at 80 characters.
+
+## Updating CI Jobs
+
+CI jobs for the latest official Swift and Xcode releases should be kept
+up to date based on the available Xcode versions that can be found
+in the [actions/virtual-environments](https://github.com/actions/virtual-environments/blob/main/images/macos/macos-10.15-Readme.md#xcode)
+repo.