Complete Exercise 6-18: Porting to Python 3

Author: schedutron

Chap6/print_vs_print_function.md

@@ -0,0 +1,3 @@
+The difference can be shown in a pretty straightforward way:
+`print(2, 3)` in Python2 outputs `(2, 3)`, whereas `print(2, 3)` in Python3
+outputs `2 3`.

README.md

@@ -85,7 +85,8 @@
     * Exercise 6-14: To be done later; same reason as above.
     * Exercise 6-15: Supporting Different RDBMSs ([ushuffle_sad.py][6-15])
     * Exercise 6-16: Importing and Python ([builtins_explanation.md][6-16])
-    * Exercise 6-17: Porting to Python3 (['warn\_vs\_print.md][6-17])
+    * Exercise 6-17: Porting to Python3 ([warn\_vs\_print.md][6-17])
+    * Exercise 6-18: Porting to Python3 ([print\_vs\_print_function.md][6-18])
 
 [req2]: /requirements.txt
 [chap4]: /Chap4
@@ -152,4 +153,5 @@
 [6-12]: /Chap6/ushuffle_dbU.py
 [6-15]: /Chap6/ushuffle_sad.py
 [6-16]: /Chap6/builtins_explanation.md
-[6-17]: /Chap6/warn_vs_print.md
+[6-17]: /Chap6/warn_vs_print.md
+[6-18]: /Chap6/print_vs_print_function.md