Disc brakes on leader doberman which has no tabs and adaptors
Why do you want to install rear discs? Keep in mind a decent set of disc brakes costs as much as this entire bike. The cost of making any improvements to this model bike is almost certainly not worth it.  

Best advise I can give is just do not go there. It results in an expensive, unreliable and often dangerous solution to a problem that does not exist in a majority of cases. If you want a better bike, sell it and buy a better bike. 

Even if you can put an adapter on to the frame, it affects wheel removal and makes all maintenance task twice as hard. You need a new hub - which practically means a new rear wheel. You won't be able to buy a rear wheel for less than the bike is worth second hand.  

90% (100% if you read Sheldon Brown) of your braking comes from your front brake. A good V Brake will brake as effectively as a low end disc brake in 90% of conditions, and on the rear wheel, won't make a significant difference in the other 10% of conditions.