character vector, mandatory!
Unpaywall requires your email address,
so that they can track usage and notify you when something breaks.
Set email address in your `.Renviron` file with
the option
`roadoi_email` options(roadoi_email = "najko.jahn@gmail.com")
.
You can open your `.Renviron` file calling `file.edit("~/.Renviron")`.
Save the file and restart your R session. To stop sharing your email
when using rcrossref, delete it from your `.Renviron` file.