add guided read cache and update tuning docs

examples/client_test.cpp

@@ -771,6 +771,7 @@ int main(int argc, char* argv[])
 	//settings.announce_to_all_trackers = true;
 	settings.optimize_hashing_for_speed = false;
 	settings.disk_cache_algorithm = session_settings::largest_contiguous;
+	settings.volatile_read_cache = true;
 
 	int refresh_delay = 1;