CHANGES.md: Sectionise 0.17.0 changes
CHANGES.md: Sectionise 0.17.0 changes

Create three sections (enhancements, bug fixes, breaking changes) in
order to organise the changes for the v0.17.0 release.

As discussed in [1], the breaking changes really aren't breaking changes
to normal users.  The only exception could be the GenerateManpage
change, but even that is only important for distro maintainers.  Hence,
move all of these to the very back, such that we can mention exciting
features like the stack integration at the top instead.

Related: https://github.com/xmonad/xmonad/pull/340

[1]: https://github.com/xmonad/xmonad/pull/340#discussion_r734956547

xmonad/xmonad