copyright for manual.pdf - clarify licence

Created by: broukema

Hi Julian! :) Based on our recent discussion, it seems that you would prefer manual.pdf to be licensed under CC-BY-ND rather than the present permissive licence. It would be best that you clarify this, which is proposed in this commit/pull request for README.md

Hypothetically, this will not prevent people from converting versions of manual.pdf in older commits to text and then into LaTeX format under the previous "permissive" licence, but my prediction is that nobody will want to do that, and people will prefer to develop documentation that is clearly free licensed rather than non-free (CC-BY-ND is non-free in the software licensing sense: it prevents Derivatives).