updated documentation

tools/usc-isi/netflow-ddos/README.md

@@ -50,4 +50,7 @@ pkts in that second received by a given destination and matching a given traffic
 (e.g., DNS replies) that could be misused for attack. Last element of the vector is label, B-benign
 or A-attack. 
 
-**Note: the tool also needs maptags.txt file in this directory**
+**Note: the tool also needs maptags.txt file in this directory**
+
+**Note: this tool selects which vectors to output probabilistically, and will produce different
+outputs for different runs.**