Adding disc brake to a bike without rear disc brake mounts
It doesn't look like that bike includes the caliper mounts, and the wheels hubs do not have rotor mounts.

Converting a bike like this to disc brakes would require custom frame work, a new fork and new wheels (or at least new hubs and a wheel build).  From a financial standpoint it makes a lot more sense to buy a bike already equipped with disc brakes.

This question covers the pros and cons for different braking systems.

EDIT I stand corrected on the issue of 'required frame work' as @cbamber85 mentions below, there is a product called Brake Therapy Conversion Kit available that will allow you to mount disc brakes on your rear triangle.  Looks like the cost is about $150 USD and you would still need new hubs.