The following are 2 different approaches to adding an id/index to RDDs and 1
approach to adding an index to a DataFrame.
Add an index column to an RDD
```scala
// RDD
val dataRDD = sc.textFile("./README.md")
// Add index then set index as key in map() transformation
// Results in RDD[(Long,
RDDs, DataFrames and Datasets are all immutable. So, you cannot edit any of
these. However, the approach you should take is to call transformation
functions on the RDD/DataFrame/Dataset. RDD transformation functions will
return a new RDD, DataFrame transformations will return a new DataFrame and
