This is an R HTML vignette. The file extension is *.Rhtml, and it has to include a special comment to tell R that this file needs to be compiled by knitr:

<!--
%\VignetteEngine{knitr::knitr}
%\VignetteIndexEntry{The Title of Your Vignette}
-->

Now you can write R code chunks:

You can also embed plots, for example:

For package vignettes, you need to encode images in base64 strings using the knitr::image_uri() function so that the image files are no longer needed after the vignette is compiled. For example, you can add this chunk in the beginning of a vignette: