How to implement a column that has a list of history value?

I have implement document library that has a column name called 'Email notification'. A person uploads a document and assign responsibility to another to review it. When s/he assign the email goes to the responsible person. If the responsible person doesn't not complete his/her work. The creator of the document will assign that document again and again till the person complete it. I have setup a workflow that do all the working of sending email and update the column email notification with text 'Email has been sent on "modified date"' i.e. it tells the status of a document. I want this column to store all the dates as a history. Example If I assign a task on 10 October and reassign the same task on 15 October. I should be able to see both dates as a history.
Hope there will be a way to do this.

Many thanks