add zenodo DOI for v0.4.2 to README
add zenodo DOI for v0.4.2 to README

kbarbary/sncosmo,sncosmo/sncosmo,sncosmo/sncosmo,kbarbary/sncosmo,kbarbary/sncosmo,dannygoldstein/sncosmo,sncosmo/sncosmo,dannygoldstein/sncosmo,dannygoldstein/sncosmo