README.md in trema-0.3.20 vs README.md in trema-0.3.21

@@ -64,10 +64,14 @@
 
 2.Install Trema at the command prompt:
 
     $ sudo gem install trema
 
-3.Follow the guidelines to start developing your OpenFlow controller. You may find the following resources handy:
+3.Optional: Install Wireshark to diagnose internal data-flows.
+
+    $ sudo apt-get install wireshark
+
+4.Follow the guidelines to start developing your OpenFlow controller. You may find the following resources handy:
 
 * The [Getting Started with Trema](https://github.com/trema/trema/wiki/Quick-start).
 * The [Trema in 10 Minutes Tutorial](http://trema-10min.heroku.com/).
 * The [Trema Tutorial](http://trema-tutorial.heroku.com/).
 * The [Programming Trema Article (in Japanese)](http://gihyo.jp/dev/serial/01/openflow_sd/0007).