Update AUTHORS.txt documentation

Author: magreenblatt

AUTHORS.txt

@@ -1,6 +1,7 @@
-# This file is an addendum to the Chromium Embedded Framework AUTHORS file.
-# Names should be added to this file like so:
-# Name or Organization <email address>
+# This file is an addendum to the Chromium Embedded Framework AUTHORS file. It
+# lists authors through March 18, 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 <magreenblatt@gmail.com>
 Kai Klimke, censhare AG <kkl@censhare.com>