de chunk won’t be run but it will show up in the document (this
is great for a tutorial where you want to show someone how to install a package,
but you don’t want to reinstall it yourself every time you knit the document);
<code>eval = TRUE, echo = TRUE</code> means our code chunk will be displayed as well as run,
and its results will be displayed too (this is great for teaching purposes, when
you want to show how to write code to produce a certain result);
<code>eval = TRUE, echo = FALSE</code> means 