Add CFF file to repo for direct citation from GH

[ddsjoberg]

When you add a CFF file to the repo, you'll see a cute button so anyone can easily cite the repo.

https://citation-file-format.github.io/#:~:text=cff%20files%20are%20plain%20text,to%20correctly%20cite%20their%20software.

With that in mind, how do you feel about submitting an article about visR to JOSS at some point?
If we do submit to JOSS, we would also want to add a inst/CITATION file to specify how the repo should be referenced (e.g. using the manuscript instead of the BibTex @Manual entry. The CITATION file will also direct how the citation() results look.

[SHAESEN2]

It was something that has been on my mind. I would say, we go for it!

[ddsjoberg]

I am cleaning up my open issues, and closing a few