GitHub for Windows has really nice release notes, I like the terminology that they use (added, fixed, improved)... we might consider using this for user-facing release notes as opposed to the dev-facing changelog though.
Thoughts:
I think we should definitely link to the GitHub issue for each bug fix or feature request in the Changelog so people can review the development process.
We should separate changes into bugs, features, and enhancements (I think they call them "support" in Fabric above), either by labeling each entry or separating them into sections
Any major changes (e.g. a Ruby version change) should be noted before anything else.