Add a section about rootlets to spinalcordtoolbox.com

[valosekj]

Originally posted by @NathanMolinier in spine-generic/data-multi-subject#158 (comment):

Also, I would like to add a small comment: I see that you are using the suffix _dseg because I imagine you are using discrete segmentations (label values may go higher than 1: not binary). However, ideally, the mapping between label values and the segmented structure should be referenced somewhere (on spinalcordtoolbox.com like discs and vertebrae or directly within the dataset). So, is this mapping available somewhere ? Apart from that the filenames are good to go !