Document how to check caretStack objects

[richierocks]

caret::caretStack() includes the time it took to run inside the result, presumably just to cause our SCTs to break.

Machine Learning Toolbox Ch5Ex16 has an example of its use and good SCTs to check it. Possibly worth documenting how to check this function (maybe just a link to the course), in case anyone else falls over this.

The link is currently https://www.datacamp.com/teach/editor/223/edit/cd0e679a7f?branch=datasets-in-image, but will be merged into master shortly.