Add new Roxygen tags

Roxygen 4.0 will add @describeIn and @field as valid tags, while 3.0 has @slot.

A more permanent solution would have roxygen2 exporting the (sorted) list of valid tags, and RStudio either querying that list or reading it once (e.g. on startup)
Merge pull request #102 from kevinushey/patch-4

Add new Roxygen tags