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.
They don't *have* to be pass as Unix style file paths. One just has to escape an `\` in a Windows style path. Hence

```r
setwd('C:\users')
```

is user error but

```r
setwd('C:\\users')
```

is just fine. This is an R idiosyncrasy though owing to some of its internals and Unix heritage.