Added python-guide link

free-programming-books.md

@@ -1479,6 +1479,7 @@ See also [TeX](#tex)
 * [Python for Informatics: Exploring Information](http://www.pythonlearn.com/book.php) (2.7.5)
 * [Python for you and me](http://pymbook.readthedocs.org/en/latest/) (2.7.3)
 * [Python for you and me](http://pymbook.readthedocs.org/en/py3/) (3.x)
+* [Python Guide](http://docs.python-guide.org/en/latest/)
 * [Python Idioms](http://safehammad.com/downloads/python-idioms-2014-01-16.pdf) (PDF)
 * [Python in Hydrology](http://www.greenteapress.com/pythonhydro/pythonhydro.html) - Sat Kumar Tomer
 * [Python Koans](https://github.com/gregmalcolm/python_koans) (2.7 or 3.x)