DOC: added version.txt file that describes how atlas was generated
DOC: added version.txt file that describes how atlas was generated

Former-commit-id: 513cb7880bc8a76e5c225c12943215f00591dd3d
Former-commit-id: b83b171696c443eb6f5bb634089b807c4775b48b
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox