Update instruction for Ubuntu 13.04.

doc/install_ubuntu.txt

@@ -39,7 +39,7 @@ probably do something similar on older computer.
 Installation steps
 ~~~~~~~~~~~~~~~~~~
 
-Ubuntu 11.10/12.04/12.10:
+Ubuntu 11.10/12.04/12.10/13.04:
  1) ``sudo apt-get install python-numpy python-scipy python-dev python-pip python-nose g++ libopenblas-dev git``
  2) ``sudo pip install Theano``
 
@@ -62,6 +62,13 @@ Test the newly installed packages
  2) SciPy (~1m): ``python -c "import scipy; scipy.test()"``
  3) Theano (~30m): ``python -c "import theano; theano.test()"``
 
+NumPy 1.6.2, 1.7.0 and 1.7.1, have a bug where it mark an ndarray as
+not aligned. Theano do not support unaligned array and verify
+that. This cause one test to fail with an unaligned error with those
+version of NumPy. You can ignore that test error as at worst, your
+code will crash. If this happen, install another NumPy version to fix
+this problem. NumPy 1.6.2 is used in Ubuntu 12.10 and NumPy 1.7.1 is
+used in Ubuntu 13.04.
 
 Speed test Theano/BLAS
 ~~~~~~~~~~~~~~~~~~~~~~