expanded motivations for R lesson

I expanded the motivation of the R lesson, and put it in its own file as recommended for the new template for lessons.

Feedback and comments welcome!

PS: that was initially part of my homework for SWC instructor training.
I'm might use "expect" instead of "require" here. There is certainly more and more emphasis on this, but I can't think of any funding agencies or journals that really require this in a meaningful sense at this point.