Prepare release notes for the next public release: 1.3

git-svn-id: https://repo.thevirtualbrain.org/svn/tvb/trunk/tvb_documentation@6612 1c0e02f0-7929-43c0-8fb3-3293bf43b0d1
Merge branch 'master' of https://github.com/the-virtual-brain/tvb-documentation