I'm aware of hundreds if not thousands of projects on GitHub alone using similar ways of documenting changes with CHANGELOG-like files and regardless of their s

I'm aware of hundreds if not thousands of projects on GitHub alone using similar ways of documenting changes with CHANGELOG-like files and regardless of their size/popularity I'm not aware of the issues you both seem to be worried about. Generally people edit changelogs after pull requests have been merged or as part of the pull requests themselves. It's far easier to figure out merge conflicts if and when other versions appended to the changelog simultaneously because all changelog entries are dated (or at least they should be) or at least versioned and contain plain text. That said I agree that we should probably spend some time detailing common processes for editing and appending to a changelog in situations with concurrent branches being developed.