Link preprint in README

[sbfnk]

I think it would be good to link to the arXiv preprint in the README.

[nikosbosse]

Agree - although probably we would want to link to the updated manuscript? The PDF is currently gitignored, so there is no really good place to link to. Alternatively we could even think about just making it a vignette completely?

[seabbs]

I think listing to the paper of record i.e. the preprint on the archive is a good idea regardless of content!

I think duplicating the content into the documentation is probably another issue (and maybe a good idea) for the future.

[sbfnk]

Closed in #563.