docs(faq): add two FAQs about the new flag, and longitudinal studies
docs(faq): add two FAQs about the new flag, and longitudinal studies

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