Add a link to the arxiv version of the paper in the README.
Add a link to the arxiv version of the paper in the README.

The README currently has an inline citation for the paper
describing the theory for the software. Adding a direct
link to the arxiv makes it easier for people to go to the
paper, as opposed to having to go to the arxiv and search
for the paper manually.

Tomographer/tomographer,Tomographer/tomographer,Tomographer/tomographer,Tomographer/tomographer