Introduce copy by slicing, used in later chapters.

Doc/tutorial/introduction.rst

@@ -523,6 +523,12 @@ concatenated and so on::
    >>> 3*a[:3] + ['Boo!']
    ['spam', 'eggs', 100, 'spam', 'eggs', 100, 'spam', 'eggs', 100, 'Boo!']
 
+All slice operations return a new list containing the requested elements.  This
+means that the following slice returns a shallow copy of the list *a*::
+
+   >>> a[:]
+   ['spam', 'eggs', 100, 1234]
+
 Unlike strings, which are *immutable*, it is possible to change individual
 elements of a list::