As much as I love Fravo, I am not sure why this feature got cut from the timeline view. Milestones are super important to have for projects - deliverables, key dates, internal reviews - you name it.
Please consider adding a milestone option in the timeline view that can be opened as a card (to take full use of the text editor).
My thoughts would be to drag and drop a pin or diamond that can sit on top of other cards for clarity.