Adding workflow for rhub

[kylebaron]

See https://r-hub.github.io/rhub/#setup

[kyleam]

Okay, so as far as I understand

• this is a manual workflow that can be triggered by calling rhub_check
• once you call that, a corresponding section will show up at https://github.com/metrumresearchgroup/mrgsolve/actions/workflows/rhub.yaml (example from cli repo)
• but you'll need to wait for rhub.yaml to hit main before you can actually try that out

[kylebaron]

Thanks; I had been using the old rhub interface when testing for CRAN, but looks like you can't do that any more. Will give this a try.