I've taken the time to fill out the changelog for 0.6.0 based on what has already been completed / what is very nearly completed.
You can find it here if you're interested: https://fossbilling.org/docs/changelog#060-wip
It's still a WIP release and we don't have an ETA on it yet, however it is getting close.
Given how long it's been since 0.5.5 and that we've recently fixed some problematic issues, it is likely that a 0.5.6 release will be made fairly soon that contains just the bug-fixes and maybe one or two of the improvements from 0.6.0.
Thanks for the interest in FOSSBilling.