Hi,
You simply need to compare the mouse position with the screen sizes and for example is within 10 pixels of the edges ( that is less then 10 if on the left side or within screen.width-10 for the right side), then you can trigger.
You could also then use the distance from the edge to vary the speed of rotation.
Mess with this, if you have problems implement this, let me know, I'll do a prefab for this, or maybe an action, seems quite useful to know the distance from the edges actually, I can see lots of usage for it.
Bye,
Jean