add references for expanded model and censoring thresholds
add references for expanded model and censoring thresholds

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