Add link to idiomatic exercise

Author: jerry-git

README.md

@@ -53,6 +53,7 @@ Python is a powerfull language which contains many language features that are no
 1. [Idiomatic dictionaries](http://htmlpreview.github.com/?https://github.com/jerry-git/learn-python3/blob/master/notebooks/intermediate/html/idiomatic_dicts.html) [[notebook]](http://nbviewer.jupyter.org/github/jerry-git/learn-python3/blob/master/notebooks/intermediate/idiomatic_dicts.ipynb)
 1. [Idiomatic Python - miscellaneous part 1](http://htmlpreview.github.com/?https://github.com/jerry-git/learn-python3/blob/master/notebooks/intermediate/html/idiomatic_misc1.html) [[notebook]](http://nbviewer.jupyter.org/github/jerry-git/learn-python3/blob/master/notebooks/intermediate/idiomatic_misc1.ipynb)
 1. [Idiomatic Python - miscellaneous part 2](http://htmlpreview.github.com/?https://github.com/jerry-git/learn-python3/blob/master/notebooks/intermediate/html/idiomatic_misc2.html) [[notebook]](http://nbviewer.jupyter.org/github/jerry-git/learn-python3/blob/master/notebooks/intermediate/idiomatic_misc2.ipynb)
+1. Idiomatic Python exercise [[exercise]](http://nbviewer.jupyter.org/github/jerry-git/learn-python3/blob/master/notebooks/intermediate/exercises/idiomatic_python_exercise.ipynb)
 
 #### General topics
 1. [Goodies of the Standard Library - part 2](http://htmlpreview.github.com/?https://github.com/jerry-git/learn-python3/blob/master/notebooks/intermediate/html/std_lib2.html) [[notebook]](http://nbviewer.jupyter.org/github/jerry-git/learn-python3/blob/master/notebooks/intermediate/std_lib2.ipynb)