COPYING: added notice about BLAKE2

COPYING

@@ -24,3 +24,15 @@ PYBLAKE2 IS OCTUPLE-LICENSED UNDER THE FOLLOWING LICENSES
 * MIT License - https://opensource.org/licenses/MIT
 * The BSD 3-Clause License - https://opensource.org/licenses/BSD-3-Clause
 * ISC License - https://opensource.org/licenses/ISC
+
+---
+
+This package includes code from the BLAKE2 project, which is only
+triple-licensed: "All code is triple-licensed under the CC0, the OpenSSL
+Licence, or the Apache Public License 2.0, at your choosing" and carries this notice:
+
+  Copyright 2012, Samuel Neves <sneves@dei.uc.pt>.  You may use this under the
+  terms of the CC0, the OpenSSL Licence, or the Apache Public License 2.0, at
+  your option.  The terms of these licenses can be found at:
+
+See https://github.com/BLAKE2/BLAKE2