Add standard `R CMD check` workflow

[mihaiconstantin]

This PR adds the R CMD check standard workflow described here. The idea is that pushes and merges into the master branch will be automatically checked using R CMD check --as-cran on three different operating systems and versions of R.

[mihaiconstantin]

Oh, I also added a basic README.md with some badges, just because I thought they look cool!

[SachaEpskamp]

Hi Mihai,

Thanks for this and sorry for the late action... I am not entirely sure what is happening here, and I prefer to just run R CMD CHECK before submitting to CRAN rather than on every Github commit, but I will give this a try, thanks!

[mihaiconstantin]

Hi Sacha,

Ah, the workflows have been updated in the meantime. Let me send another PR with the corresponding updates!

About what it does, the idea is that every time you merge/ commit to the main or mater branch, it runs the R CMD check --as-cran on three operating systems and three versions of R I think (i.e, old release, release, and the development branch). It can be handy to catch things before the CRAN team...

For public repositories, you have a certain number of run minutes (i.e., 3000 I believe), but if you associate your .edu address with GitHub (i.e., this page), you essentially have unlimited run times.

I'll follow up shortly with the updated workflows!

Best, Mihai

[SachaEpskamp]

Thanks a lot! I will handle the next PR faster haha!