Change copyright to The OpenThread Authors. (#583)

- Add a top-level file named AUTHORS which identifies the copyright
holders of the project.

AUTHORS

@@ -0,0 +1,11 @@
+# This is the official list of OpenThread authors for copyright purposes.
+#
+# This does not necessarily list everyone who has contributed code, since in
+# some cases, their employer may be the copyright holder. To see the full list
+# of contributors, see the revision history in source control or
+# https://github.com/openthread/openthread/graphs/contributors
+#
+# Authors who wish to be recognized in this file should add themselves (or
+# their employer, as appropriate).
+
+Nest Labs, Inc.