Adding caveats about setwd() to R novice lesson

For our final instructor training assignment, @sritchie73 and I have added some caveats about the use of `setwd()` in R scripts to the Best Practices section of the novice R lessons. We point out that `setwd()` can cause problems if others try to run your code on their machines, and we provide an example of the type of error that can result.
Directories have to be passed with unix style '/' in windows