Add a note about the openblas package that could conflict with atlas.

doc/install_ubuntu.txt

@@ -43,6 +43,12 @@ Ubuntu 11.10/12.04/12.10:
  1) ``sudo apt-get install python-numpy python-scipy python-dev python-pip python-nose g++ libopenblas-dev git``
  2) ``sudo pip install Theano``
 
+ If the packages ``libatlas3gf-base`` or ``libatlas-dev`` are already installed, there will be problems as they conflict with ``libopenblas-dev``.
+ If you see NumPy errors, the simplest is to remove ``libopenblas-dev`` and its dependency ``libopenblas-base`` like this: ``sudo apt-get remove libopenblas-base``.
+ The ideal would be that you remove ``libatlas3gf-base`` and ``libatlas-dev``,
+ but you will need to reinstall python-numpy, python-scipy and all other packages that used it.
+ openblas is faster then ATLAS most of the time and it allow to control the number of thread used during the execution.
+
 Ubuntu 11.04:
  1) ``sudo apt-get install python-numpy python-scipy python-dev python-pip python-nose g++ git libatlas3gf-base libatlas-dev``
  2) ``sudo pip install Theano``