Prepare repo for going public

[mih]

• [x] create a README that explain how to compile the paper, and that this also actually performs the analysis -- if DataLad is around. Confirm that the entire manuscript can be built from a plain clone of this repo, plus the steps given in the README.
• [x] beautify/truncate Git history and force-push master -- decision to not do that
• [x] Add license specification (same as biorxiv)
• [x] make public once the preprint is uploaded
• [x] double check and resolve remaining issues at https://github.com/psychoinformatics-de/studyforrest-data-eyemovementlabels/issues