update README

Author: liyuliang001

traffic_gen/README.md

@@ -14,3 +14,6 @@ The generate traffic can be directly used by the simulation.
 The first line is the number of flows.
 
 Each line after that is a flow: `<source host> <dest host> 3 <dest port number> <flow size (bytes)> <start time (seconds)>`
+
+## Flow size distributions
+We provide 4 distributions. `WebSearch_distribution.txt` and `FbHdp_distribution.txt` are the ones used in the HPCC paper. `AliStorage2019.txt` are collected from Alibaba's production distributed storage system in 2019. `GoogleRPC2008.txt` are Google's RPC size distribution before 2008.