Explain that require(S7) will cause R CMD Check WARNING & NOTE

[jonthegeek]

https://github.com/RConsortium/OOP-WG/blob/965ab8ff1da9a599d494ed10dee033fa27a99e4c/vignettes/packages.Rmd#L77

The wording here seems a bit too gentle. It generates a WARNING and a NOTE in R CMD Check (rightly), which might be more than users expect.

* checking dependencies in R code ... WARNING
Warning: 'library' or 'require' call not declared from: ‘S7’
'library' or 'require' call to ‘S7’ in package code.
  Please use :: or requireNamespace() instead.
  See section 'Suggested packages' in the 'Writing R Extensions' manual.

* checking R code for possible problems ... NOTE
File ‘rapid/R/zzz.R’:
  .onAttach calls:
    require(S7)
Package startup functions should not change the search path.
See section ‘Good practice’ in '?.onAttach'.