add documentation on using a custom fitter or sampler
add documentation on using a custom fitter or sampler

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