updated CHANGES for 3.1
updated CHANGES for 3.1


Former-commit-id: 416c2ccd984b084acaf0556f0ca52baeda85df00
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox