In the Worksheet you can add this and it should do what you want.
As a way to get started.
You could additionally capture the existing comments if you like and append new comments to them, if you need to keep track of all changes.(that is not reflected here, but you might try that as well)
Public aCmt As String
Private Sub Worksheet_Change(ByVal Target As Range)
If ActiveCell.Text <> aCmt Then
With ActiveCell
If .Comment Is Nothing Then
ActiveCell.AddComment
ActiveCell.Comment.Text Text:=aCmt
Else
ActiveCell.Comment.Text Text:=aCmt
End If
End With
End If
End Sub
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
aCmt = ActiveCell.Text
End Sub