Update NEWS.txt.

NEWS.txt

@@ -21,6 +21,8 @@ Interface change:
 
 New features:
  * Many infer_shape implemented on sparse matrices op. (David W.F.)
+ * Added theano.sparse.verify_grad_sparse to easily allow testing grad of
+  sparse op. It support testing the full and structured gradient.
  * The keys in our cache now store the hash of constants and not the constant values
    themselves. This is significantly more efficient for big constant arrays. (Frederic B.)
  * 'theano-cache list' lists key files bigger than 1M (Frederic B.)