We are switching to a new system for preparing release notes.
* Release notes will be compiled when creating a release candidate from
all the commits that are part of that release.
* PRs can include suggestions for release notes in PR messages. Please
use language such as "release notes" and "breaking changes" to call
attention to the suggestions. Release notes are user-centric rather than
developer-centric.
* For more complicated release notes, these can be put into the
`releases_drafts` folder.
This solves an issue where PRs that include release notes can, when
merged, have those notes appear under the wrong Lean version, since they
might have been created before a release but not merged until after. It
also solves merge conflicts due to multiple PRs updating the release
notes.
