Modify README.txt in derivatives to explain processing

# Context

Related to #123, the links in the /derivatives/labels_softseg needs to be modify according to the new merged processing pipeline (not merged in master yet in https://github.com/sct-pipeline/contrast-agnostic-softseg-spinalcord/blob/sb/merge_processing/process_data.sh) and a new README in /derivatives/labels also needs to be added to detail how the segmentations were created.