Merge pull request scikit-learn#5834 from hugobowne/dev_requirement_documentation

[MRG]added cython to requirements in documentation

Author: amueller

README.rst

@@ -49,7 +49,8 @@ scikit-learn is tested to work under Python 2.6, Python 2.7, and Python 3.4.
 `six <http://pythonhosted.org/six/>`_). It should also work with Python 3.3.
 
 The required dependencies to build the software are NumPy >= 1.6.1,
-SciPy >= 0.9 and a working C/C++ compiler.
+SciPy >= 0.9 and a working C/C++ compiler. For the development version,
+you will also require Cython >=0.23.
 
 For running the examples Matplotlib >= 1.1.1 is required and for running the
 tests you need nose >= 1.1.2.

doc/developers/advanced_installation.rst

@@ -366,7 +366,8 @@ bleeding edge
 =============
 
 see section :ref:`git_repo` on how to get the development version. then follow
-the previous instructions to build from source depending on your platform.
+the previous instructions to build from source depending on your platform. 
+You will also require Cython >=0.23 in order to build the development version.
 
 
 .. _testing: