Update README.asc
Update README.asc

Point people to ASF Slack rather than IRC

Closes #498

bdeggleston/cassandra,jbellis/cassandra,mshuler/cassandra,jeromatron/cassandra,beobal/cassandra,driftx/cassandra,stef1927/cassandra,szhou1234/cassandra,adejanovski/cassandra,driftx/cassandra,thelastpickle/cassandra,jrwest/cassandra,mike-tr-adamson/cassandra,tjake/cassandra,instaclustr/cassandra,stef1927/cassandra,thelastpickle/cassandra,szhou1234/cassandra,mshuler/cassandra,krummas/cassandra,blerer/cassandra,krummas/cassandra,tjake/cassandra,mike-tr-adamson/cassandra,blambov/cassandra,snazy/cassandra,jbellis/cassandra,adelapena/cassandra,jeromatron/cassandra,michaelsembwever/cassandra,tjake/cassandra,blambov/cassandra,blambov/cassandra,jeromatron/cassandra,michaelsembwever/cassandra,mike-tr-adamson/cassandra,beobal/cassandra,bdeggleston/cassandra,adelapena/cassandra,yukim/cassandra,pauloricardomg/cassandra,snazy/cassandra,krummas/cassandra,yukim/cassandra,pauloricardomg/cassandra,blerer/cassandra,mshuler/cassandra,driftx/cassandra,jasonstack/cassandra,adelapena/cassandra,belliottsmith/cassandra,mike-tr-adamson/cassandra,szhou1234/cassandra,blerer/cassandra,jbellis/cassandra,adejanovski/cassandra,adejanovski/cassandra,michaelsembwever/cassandra,krummas/cassandra,szhou1234/cassandra,stef1927/cassandra,belliottsmith/cassandra,apache/cassandra,yukim/cassandra,instaclustr/cassandra,instaclustr/cassandra,pauloricardomg/cassandra,jasonstack/cassandra,michaelsembwever/cassandra,adelapena/cassandra,belliottsmith/cassandra,beobal/cassandra,blambov/cassandra,blerer/cassandra,jasonstack/cassandra,thelastpickle/cassandra,driftx/cassandra,yukim/cassandra,apache/cassandra,apache/cassandra,bdeggleston/cassandra,beobal/cassandra,pauloricardomg/cassandra,tjake/cassandra,instaclustr/cassandra,snazy/cassandra,snazy/cassandra,bdeggleston/cassandra,thelastpickle/cassandra,jrwest/cassandra,belliottsmith/cassandra,mshuler/cassandra,apache/cassandra,stef1927/cassandra,jrwest/cassandra,jrwest/cassandra