Add cython version note to README.

README.md

@@ -24,6 +24,17 @@ pip install git+https://github.com/lucasb-eyer/pydensecrf.git
 
 and ignoring all the warnings coming from Eigen.
 
+Note that you need a relatively recent version of Cython (0.23.4 at the time of writing) for this wrapper,
+the one shipped with Ubuntu 14.04 is too old. (Thanks to Scott Wehrwein for pointing this out.)
+You can replace it by a newer version either by
+
+```
+sudo apt-get remove cython
+sudo pip install -U cython
+```
+
+or (recommended) by using a [virtual environment](https://virtualenv.readthedocs.org/en/latest/).
+
 Usage
 =====