Add in the license file that we have CnMeM with the same lisence and add its copyright

doc/LICENSE.txt

@@ -9,6 +9,9 @@ All rights reserved.
 Contains code from NumPy, Copyright (c) 2005-2011, NumPy Developers.
 All rights reserved.
 
+Contain CnMeM under the same license with this copyright:
+Copyright (c) 2015, NVIDIA CORPORATION. All rights reserved.
+
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are met: