Added install instructions for Python 3 in Ubuntu

doc/install_ubuntu.txt

@@ -19,6 +19,13 @@ For Ubuntu 11.10 through 14.04:
     sudo apt-get install python-numpy python-scipy python-dev python-pip python-nose g++ libopenblas-dev git
     sudo pip install Theano
     
+On 14.04, this will install Python 2 by default. If you want to use Python 3:
+
+.. code-block:: bash
+
+    sudo apt-get install python3-numpy python3-scipy python3-dev python3-pip python3-nose g++ libopenblas-dev git
+    sudo pip install Theano
+
 For Ubuntu 11.04:
 
 .. code-block:: bash