add using roxygen2's @include to next upkeep list

[topepo]

tidymodels (and ggplot2) have files named aaa_{blah}.R to control the source order.

We should remind folks to use better file names by adding @include in the relevant files.

[jennybc]

Do "we" have some sort of official stance on this? I don't object or anything, it's just the first time I've come across this as a concrete suggestion.