Update README

Author: gjbex

source-code/object-orientation/README.md

@@ -21,3 +21,4 @@ Some examples of object-oriented programming in Python.
   * `mix-ins`: illustration of mix-ins.
   * `finite-state-parser`: illustration of object-oriented data
     representation.
+  * `attr_intro.ipynb`: illustration of how to use the `attrs` package.