MediaWiki is currently developed in two branches, with everything going to trunk, and a slower number of revisions getting also copied to the stable branch. That's also the way we want to continue working, having a trunk and a stable branch.
Currently we store the release notes in a file called RELEASE-NOTES in the phase3 root of each branch. This is a problem when adding a revision for backport, since the release notes are not suitable to be added to the trunk RELEASE-NOTES (they would go inside HISTORY) but it needs to be added to REL1_X RELEASE-NOTES. So we end up with release notes added in a different commit, or revisions with merge conflicts for merging. Which is inconvenient.
Thus, I propose that, from the point we branch 1.18, we keep the stable branch release notes in trunk, and add trunk release notes in a different file. trunk and branch RELEASE-NOTES would effectively be the same file (a revision modifing RELEASE-NOTES and not tagged for backport would be a bug). This simple change would give us much cleaner merges.
When tagging a new branch, the trunk RELEASE-NOTES would move to HISTORY, the trunk release notes file to RELEASE-NOTES and a new one would be created for trunk.