Add link to a blog that explain how to install Theano on ubuntu with PPA.

doc/install.txt

@@ -79,6 +79,11 @@ Linux
 
 .. _linux_basic:
 
+Alternative installation on Ubuntu with PPA
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+One user provided some `installation documentation <http://www.benmccann.com/dev-blog/installing-cuda-and-theano/>`_ for Ubuntu 11.04 with CUDA 4.0 PPA.
+
 Basic user install instructions
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~