Check that package uses `Authors@R` in `DESCRIPTION`

[ateucher]

CRAN recently requires that the DESCRIPTION file specifies authors using the Authors@R field. Also, #243 assumes this is the case

[mpadge]

Thanks @ateucher, looking forward to a PR here. Feel free to ask any questions along the way. FYI @maelle This is one of the new check ideas that came from last week's pkgcheck workshop 🚀

[maelle]

Awesome! Besides, the rOpenSci dev guide requires usage of the field.