Apologies for being late to the dance.
I am sorry to hear about this.
I will assume that the injury was evaluated and fracture and separated cartilage was ruled out.
Where the injury is has a lot to do with what helps/hurts recovery.
Rest is usually advised, but if breathing is aggravating you can't very well stay off your lungs.
If you practice any kind of breathing exercises it is a good idea to start with that as soon as possible. Zen breathing/japanese breathing exercises are usually pretty compatible with judo training. Proper breathing, "abdominal" breathing, is necessary for all kinds of muscle function and stabilization and rib injuries can screw with it pretty badly. Make sure this isn't happening, plus it gives you something to work on while you are down. Any pre-hab/re-hab that you "know" you should be doing but brush off for harder/cooler training is also a great idea. Catch up on it now so you can hit randori with a vengeance when its time.
If the bruise is more anterior foam rolling your spine/back muscles can help mobilize your ribs and may speed healing, or at least speed getting back into form later.
I am a big fan of using liniment to help with bruises because it both relieves pain and increases circulation to the injury. Of course bones are deep to the skin so this may not be of much value other than as an analgesic.
Tape/ wrapping the rib before provocative/pain causing activities is an old stand by.
General rule is to not push any activity that makes you suffer for longer than 10 minutes after you stop. So if you are still in more pain after resting for ten minutes, go easier or do something else. If it only sucks more for 5 minutes after you are done than you are probably ok.
Once again I am taking you at your word that nothing more serious is causing the pain. Nothing I wrote is a substitute for actual medical evaluation and advice.