Document how to segment a folder

[jcohenadad]

There is currently no documentation about how to use the tool. In order to provide feedback to the dev team, a minimalist documentation is required. Below is an example scenario for which a documentation would be a good starting point:

• Select a folder that has BIDS-organize data
• At the root, there is a folder /derivatives/labels, that contains labels for some subjects.
• The data of interest have the suffix _T1w.nii.gz and the labels of interest (the existing ones, and those to create from scratch) have the suffix _T1w_label-SC_seg.nii.gz
• Select a value to assign to the label (eg: 1)
• Start the segmentation in a few subjects