Added link in the Theano tutorial to scan.

doc/tutorial/index.txt

@@ -32,6 +32,7 @@ you out.
     symbolic_graphs
     modes
     aliasing
+    loop
     using_gpu
     pycuda
     shape_info

doc/tutorial/loop.txt

+.. _tutloop:
+
+====
+Loop
+====
+
+You can use :ref:`Scan <lib_scan>` to do all type of loop in Theano. All the documentation about it is in the library for now.