Small note to describe Nil

This is the first time the user comes across Nil in the tutorial, so a small explanation of what it is.

Author: manishpatelUK

_tour/generic-classes.md

@@ -31,6 +31,8 @@ class Stack[A] {
 ```
 This implementation of a `Stack` class takes any type `A` as a parameter. This means the underlying list, `var elements: List[A] = Nil`, can only store elements of type `A`. The procedure `def push` only accepts objects of type `A` (note: `elements = x :: elements` reassigns `elements` to a new list created by prepending `x` to the current `elements`).
 
+`Nil` here is a special case of `List` and is not to be confused with `Null`. It is a `List` of zero length.
+
 ## Usage
 
 To use a generic class, put the type in the square brackets in place of `A`.