Add an `AUTHORS` file and update copyright notices.

Change-Id: Ifb505e5664996c1af41e38376e58ba49864213a3
diff --git a/.clang-format b/.clang-format
index 0a7e60c..31e0b6e 100644
--- a/.clang-format
+++ b/.clang-format
@@ -1,4 +1,4 @@
-# Copyright 2016, ARM Limited
+# Copyright 2016, VIXL authors
 # All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without