Add "prerequisites" for course

[joelnitta]

This course assumes moderate proficiency in R, especially the ability to write functions.

A disclaimer about prerequisites should probably be added to the Summary, possibly with language similar to the {targets} README:

Prerequisites
Familiarity with the R programming language, covered in R for Data Science.
Data science workflow management techniques.
How to write functions to prepare data, analyze data, and summarize results in a data analysis project.