I'm assigned to a project where the goal is to update an existing software. This software was developed in a totally ad hoc manner, which means that any documentation generated is outdated, confused and just plain useless. I do not want to retroactivelly produce all the missing artifacts, since this would just be a waste of time. The code base is ugly, and really need some serious refactoring, but this is another story.

What I want is to apply Readme Driven Development to guide this project. So, basically, I want to write a Readme, which will document every change meaningful at the user level. The problem is, I have no idea about how to structure this document. For me, it's pretty clear that it should have a different layout than the traditional Readme. I really want to document changes, and not the current state of affairs.

Any ideas?