DOC: added info on usage
DOC: added info on usage

Former-commit-id: 177231427eac4217d2a891d5f88015539f50ccd3
Former-commit-id: bc71b1b7bf1c69a670c904eee8a4bbf8d4ca1d7e
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox