Adding AUTHORS and changing copyright headers to refer to it.

Change-Id: I3c2b83e7cd276fd875a497047c5c4f270625e856
diff --git a/BUILD.gn b/BUILD.gn
index 3e5491e..2bd65ed 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1,4 +1,4 @@
-# Copyright 2018 Google LLC
+# Copyright 2018 The Hafnium Authors.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.