use_gpl3_license does not create GPL3+ file LICENSE like MIT

[sriharitn]

When we use, use_gpl3_license() the description file does not contain CRAN compliant License: GPL-3 + file LICENSE in description file.

Without this feature, when we run R CMD check --as-cran mypackage we get the following NOTE:

* checking top-level files ... NOTE
File
  LICENSE

Having use_gpl3_license() output GPL-3+ file LICENSE in description file would be very helpful similar to the output of use_mit_license()

[jennybc]

Would you like to fix this in a pull request?

[sriharitn]

Yes please that would be great.

[hadley]

LICENSE should automatically be added to .Rbuildignore and the existing code looks correct to me.

[jennybc]

I consider this fair game for tidyverse dev day. Removing the WIP label.

[jennybc]

Question from Cervan: Should I do this for other licenses while I'm working on it?

Answer: Search in Writing R Extensions to see if GPL-3 is the only one that needs this treatment (besides MIT).