NEW YORK, Nov 6 (Reuters) - U.S. newspaper publishers Gannett Co. Inc. (GCI.N: Quote, Profile , Research), McClatchy Co. (MNI.N: Quote, Profile , Research) and Tribune Co. (TRB.N: Quote, Profile , Research) have invested an additional $15 million in Topix.net, the news search and archive company said on Monday. The Palo Alto, California-based Topix plans to use the money to expand its news...