Update LICENSE.txt (#450)

[ROCm/rccl commit: 04c10a6025]
2021-11-05 20:43:53 +05:30
@@ -1,6 +1,11 @@

- Copyright (c) 2015-2020, NVIDIA CORPORATION. All rights reserved.
- Modifications Copyright (c) 2019-2020 Advanced Micro Devices, Inc. All rights reserved.
+Attributions
+
+Contains contributions from NVIDIA.
+
+Copyright (c) 2015-2020, NVIDIA CORPORATION. All rights reserved.
+copyright = 'Copyright 2019-2021 Advanced Micro Devices'.
+2015-2018, NVIDIA CORPORATION; Modifications Copyright 2019-2021 Advanced Micro Devices.

 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions