update NOTICE for for third-party libs

Patch by eevans; reviewed by jbellis for CASSANDRA-411


git-svn-id: https://svn.apache.org/repos/asf/incubator/cassandra/branches/cassandra-0.4@810281 13f79535-47bb-0310-9956-ffa450edef68

NOTICE.txt

@@ -5,3 +5,10 @@ This product includes software developed by The Apache Software
 Foundation (http://www.apache.org/).
 
 Developers and Contributors are listed in the project POM file
+
+JSON support is provided by Flexjson (http://flexjson.sf.net).
+Copyright 2007 Charlie Hubbard.
+
+This product makes use of the google-collections library from
+http://code.google.com/p/google-collections/.
+Copyright (C) 2008 Google Inc.