Allow contributor to leave a comment to the task provider

[joostschouppe]

Sometimes, it is really useful to get some feedback from the mapper. E.g. you leave them a task to fix, but they notice something related to the challenge is pretty weird. It would be nice if there was a free text field where the mapper can leave a message like this. That way, a task manager can more easily have a closer look at fixed-but-special tasks.