Add extending `R` with your own functions to Setup chapter

[agarbiak]

Add section about extending R with your own functions:

CS1/setup.Rmd

Line 138 in a064d0a

## Extending `R` with packages

Something like:

## Extending `R` with your own functions

We can further "_extend_" `R` by implementing our own functions:

```{r setup-udfs}
# UDF example
```