Add notebook that illustrates __format__

Author: gjbex

source-code/object-orientation/README.md

@@ -20,3 +20,7 @@ Some examples of object-oriented programming in Python.
     of multiple inheritance in Python.
   * `mix-ins`: illustration of mix-ins.
   * `attr_intro.ipynb`: illustration of how to use the `attrs` package.
+  * `f_string_user_defined_formats.ipynb`: illustration of how to use
+    f-strings with user-defined classes by overriding the `__format__`
+    method.  It also illustrates the use of `__new__` to create
+    instances of a class while checking for validity of the arguments.