Change copyright to The OpenThread Authors. (#583)

- Add a top-level file named AUTHORS which identifies the copyright
holders of the project.
diff --git a/examples/platforms/cc2538/Makefile.am b/examples/platforms/cc2538/Makefile.am
index 1416d48..cb2118f 100644
--- a/examples/platforms/cc2538/Makefile.am
+++ b/examples/platforms/cc2538/Makefile.am
@@ -1,5 +1,5 @@
 #
-#  Copyright (c) 2016, Nest Labs, Inc.
+#  Copyright (c) 2016, The OpenThread Authors.
 #  All rights reserved.
 #
 #  Redistribution and use in source and binary forms, with or without