Merge pull request #1555 from sciunto/user_guide-joblib

DOC: Add link joblib website in userguide

doc/source/user_guide/tutorial_parallelization.txt

@@ -3,7 +3,9 @@ How to parallelize loops
 ========================
 
 In image processing, we frequently apply the same algorithm
-on a large batch of images. Here is an example:
+on a large batch of images. In this paragraph, we propose to
+use `joblib <https://pythonhosted.org/joblib/>`_ to parallelize
+loops. Here is an example of such repetitive tasks:
 
 .. code-block:: python