I was diagnosed 30 years ago, when i just started riding. Bicycling really helps. Keeps my lungs open. Pace yourself so you don't trigger exercise induced asthma. Work on increasing lung capacity. (You will!) Relax on the bike. Learn how to breath from your abdomen if you don't now (most people don't. Put your hand on your belly and take a deep breath. Your belly should expand, not your upper chest). Maybe something like tai chi or yoga would help that.