See this thread: https://productforums.google.com/forum/#!category-topic/docs/how-do-i/_oFmvMZUCqg
In particular this script:
function onEdit(e){
// Set a comment on the edited cell to indicate when it was changed.
var range = e.range;
range.setNote('Last modified: ' + new Date());
}
Also found here: https://developers.google.com/apps-script/guides/triggers/#onedit
I'm looking to make this happen only for cells in a certain column, and not sure how to modify the script accordingly (or if I should take a different approach).