Added doc about sparse sandbox op.

doc/library/index.txt

@@ -17,6 +17,7 @@ Types and Ops that you can use to build and compile expression graphs.
    printing
    compile/index
    sparse/index
+   sparse/sandbox
    scalar/index
    gof/index
    scan

doc/library/sparse/sandbox.txt

+..  ../../../../theano/sparse/sandbox/sp.py
+..  ../../../../theano/sparse/sandbox/truedot.py
+
+.. _libdoc_sparse_sandbox:
+
+===================================================================
+:mod:`sparse.sandbox` --  Sparse Op Sandbox
+===================================================================
+
+.. module:: sparse.sandbox
+   :platform: Unix, Windows
+   :synopsis: Sparse Op Sandbox
+.. moduleauthor:: LISA
+
+API
+===
+
+.. automodule:: theano.sparse.sandbox.sp
+    :members:
+.. automodule:: theano.sparse.sandbox.truedot
+    :members: