Add warning about old-style classes

Akuli · web-flow · commit 0a12c33ef436 · 2016-07-15T18:41:20.000+03:00
diff --git a/classes.md b/classes.md
@@ -63,6 +63,13 @@ Let's use it to define an empty class.
 >>> 
 ```
 
+**_Note_:** If you are using Python 2 I highly recommend using
+`class Website(object):` instead of `class Website:`. This creates a
+new-style class instead of an old-style class. Old-style classes are
+different than new-style classes in some ways and not supported in this
+tutorial. In Python 3, there are no old-style classes and
+`class Website(object):` does the same thing as `class Website:`.
+
 Note that I named the class `Website`, not `website`. This way we know
 that it's a class. Built-in classes use lowercase names (like `str`
 instead of `Str`) because they are faster to type, but use CapsWord
