Although we’ve covered more than enough to get you quite far using R markdown, as with most things R related, we’ve really only had time to scratch the surface. Happily, there’s a wealth of information available to you should you need to expand your knowledge and experience. A good place to start is the excellent free book written by the creator of R markdown Yihui Xie which you can find here.
Another useful and concise R markdown reference guide can be found here
A quick and easy RStudio R markdown cheatsheet