Qualify when re-installation is needed

[eddelbuettel]

As mentioned in this tweet to you which you may have missed, it may be beneficial to state that these reinstallations are not always required on a R x.y.0 installation: While R 4.0.0 needed, as well as one of the recent R 3.* (it may have been R 3.5.* two years before), many other "major" releases do not need it.

People have enough fear and loathing of upgrades, plus sources may have gone missing from CRAN, plus system libraries may have changed etc -- I think it is beneficial to tell folks that they can in fact save a few million cpu cycles and not reinstall.

Thanks for putting a scripts repo together though. These are helpful.