Update AUTHORS.txt documentation

Author: magreenblatt

AUTHORS.txt

@@ -1,6 +1,7 @@
-# This file is an addendum to the Chromium AUTHORS file.
-# Names should be added to this file like so:
-# Name or Organization <email address>
+# This file is an addendum to the Chromium AUTHORS file. It lists authors
+# through March 16, 2015 when Git was introduced for source code management.
+# A list of authors after that date can be found by executing this command on
+# a local Git checkout: git log --all --format="%aN <%aE>" | sort -u
 
 Marshall Greenblatt <[email protected]>
 Jamie Kirkpatrick <[email protected]>