dcm2bids_scaffold needs a refresher

[SamGuay]

While updating the tutorial, I noticed that the dcm2bids_scaffold function could get a little bit of love.

• [ ] participants.tsv and participants.json should at least match.
• [ ] README could take some inspiration from the BIDS Starter Kit's.

In addition to these two updates, we could potentially add an optional flag for rawdata/, phenotype/ directories, and probably others I'm forgetting atm.