In CentOS6, add a bleeding edge section.

doc/install_centos6.txt

@@ -100,3 +100,8 @@ If you want to also update NumPy/SciPy, you can run this:
 
     sudo pip install --upgrade theano
 
+Bleeding edge
+~~~~~~~~~~~~~
+
+Do like in the section "Updating Theano", but use
+``git+git://github.com/Theano/Theano.git`` instead of ``theano``.