add warning for default work/output directories
add warning for default work/output directories

chrisfilo/fmriprep,shoshber/fmriprep,poldracklab/fmriprep,oesteban/fmriprep,oesteban/preprocessing-workflow,oesteban/fmriprep,poldracklab/fmriprep,shoshber/fmriprep,oesteban/preprocessing-workflow,poldracklab/fmriprep,oesteban/fmriprep,poldracklab/preprocessing-workflow,poldracklab/preprocessing-workflow