Add key event comments, that show in the stats when you hover on the graph

What?
Simple piece of data just

• Date
• Comment - free text

Comments could be "Meeting with client to review progress" or "User approved testing session", "feedback from release version 1.x"

Where?
To be shown as a milestone indicator on the burn up graph, so you can see the comment when you hover over it.

Why?
Basically the events that caused another batch of stories to be added to the backlog.
This would add a commentary to the burn up chart to answer the question "why is this chart going up?"

Who?
Anyone who cad add stories to the backlog.