Better explanation of list slices

Step size and list slice syntax was not quite clear. 

Upon seeing li[::-1] as it was previously, I was not sure what this really meant so I looked it up and found that this section of the guide may benefit from more explanation.

python.html.markdown

@@ -160,8 +160,12 @@ li[1:3] #=> [2, 4]
 li[2:] #=> [4, 3]
 # Omit the end
 li[:3] #=> [1, 2, 4]
+# Select every second entry
+li[::2] #=>[1,4]
 # Revert the list
 li[::-1] #=> [3, 4, 2, 1]
+# Use any combination of these to make advanced slices
+# li[start:end:step]
 
 # Remove arbitrary elements from a list with "del"
 del li[2] # li is now [1, 2, 3]