CHANGELOG.md on master is not up-to-date

[therealprof]

Specifically it's missing the data from the 0.6.3 release. If everything was backported then they can probably be identical otherwise we'd might be short a few "unreleased" changes, too.

[adamgreig]

A number of breaking changes were not backported; putting them into unreleased and trying to get changelog entries included in future PRs is probably worthwhile...