CI checks for changelog entries

[guseggert]

It would be awesome if we could have CI checks so that each commit either needs to add a changelog entry, or explicitly opt-out of it (e.g. with "nochangelog" in commit message). This would ensure that each commit gets a changelog entry so that there's not a scramble at release time from folks forgetting to add changelog entries.

[galargh]

This is out of scope for this project. We're going to work on custom solution for boxo/kubo separately.