docs: Release notes should be updated in a separate commit

docs: Release notes should be updated in a separate commit

Updating docs/news.xml in the same commit that performs the
documented change makes backports needlessly complicated,
both for mainteinance branches and downstream distributions,
because it introduces additional potential for merge
conflicts.

Document in the contributor guidelines that the release notes
should be updated in a separate commit instead, so that it's
easy to backport just the code change.