Documentation notebook on using model from R

[Using the `reticulate` R package](https://rstudio.github.io/reticulate/articles/calling_python.html)

Also jotting down here for future reference that [this `reticulate` vignette](https://cran.r-project.org/web/packages/reticulate/vignettes/package.html) could help us write an R package to wrap the Python package. This would be especially handy when we make the `policyengine` package that serves as the interface for all country packages, so we can just have one R package.