Make sure slides are up-to-date with new usethis workflow

[angela-li]

The R Packages book got a major overhaul, and so did the usethis package - this requires a similar update to our curriculum, to make sure we're teaching people up-to-date processes (certain things in the slides are now defunct)

See the "The Whole Game" workflow in R Packages: https://r-pkgs.org/whole-game.html

[3mmaRand]

I think we've achieved this for the York Workshop 2020

[angela-li]

I'll take a look at the materials you uploaded @3mmaRand - many thanks for putting in the hard work of updating this curriculum!