Add examples for using recipes and model formulas

[cimentadaj]

Thanks a lot for this package, it's a great addition to tidymodels! I've been following the conversation on multilevel models and tidymodels somewhere else I can't remember and I think it would be very useful to provide some documentation (vignette, README) on how to combine recipes and model formulas.

This can become a bit complex if your recipe is very long and then having to define the model formula separately.

Just leaving it here in case it's useful.

[juliasilge]

We do really need to add this kind of documentation, as folks look for guidance and can't find it. Perhaps as an "extra" non-CRAN vignette for the pkgdown site.

[topepo]

When you load multilevelmod, the parsnip man pages should update the list of engines:

(This is with recent versions of parsnip)

Those links give you more specific information about the models and how to use them. For the lmer engine, here is what that looks like (on the pkgdown site at least). There is some of that information but let us know if there are gaps.

We are working more on surfacing the documentation. It occurs to me, as I had to figure out where the information is on the pkgdown site, that we should add them to the reference section.

I think that a separate article would be great (with a worked example). This video (slides here) might also be helpful.

[juliasilge]

The censored package has the same problem and @hfrick said she is going to add links to that table in the README for engine detail pages; maybe we should do all the parsnip extension packages the same way.