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.
Windows output:

> getwd()
[1] "\\\\fsusers.mc.cumc.columbia.edu/usersd$/dyc2112/dyc2112's Profile/Documents"
> setwd('C:\users')
Error: '\u' used without hex digits in character string starting "'C:\u"
> setwd('C:/users')
> getwd()
[1] "C:/users"