Elaborate on the use of Rmd files

[wlandau]

In the main example, an Rmd file is responsible for processing the data.

> library(DataPackageR)
> processing_code <- system.file(
+   "extdata", "tests", "subsetCars.Rmd", package = "DataPackageR"
+ )
> cat(readLines(processing_code), sep = "\n")
---
title: "Test DataPackageR"
author: "Greg Finak"
output: html_document
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```

## R Markdown

This is an R Markdown document. Markdown is a simple formatting syntax for authoring HTML, PDF, and MS Word documents. For more details on using R Markdown see .

When you click the **Knit** button a document will be generated that includes both content as well as the output of any embedded R code chunks within the document. You can embed an R code chunk like this:

```{r cars}
summary(cars)
```

Note that the `echo = FALSE` parameter was added to the code chunk to prevent printing of the R code that generated the plot.

```{r}
cars_over_20 = subset(cars, speed > 20)
```

I can see how DataPackageR can grab cars_over_20 from the environment of the knitr document.

https://github.com/RGLab/DataPackageR/blob/cb23c6b7f862798706339858e81da4a004dc1712/R/processData.R#L211-L216

However, other new users may not understand this at first. I think you can elaborate on how DataPackageR finds and saves cars_over_20, and I think the right place to do it is inside the body of the above Rmd document. I am not sure you need the default text in the "R Markdown" section either.

[wlandau]

The new Rmd writeup is super thorough. Nicely done.