Add Communication intro image

Author: donnemartin

README.md

@@ -1443,3 +1443,11 @@ If queues start to grow significantly, the queue size can become larger than mem
 * [Applying back pressure when overloaded](http://mechanical-sympathy.blogspot.com/2012/05/apply-back-pressure-when-overloaded.html)
 * [Little's law](https://en.wikipedia.org/wiki/Little%27s_law)
 * [What is the difference between a message queue and a task queue?](https://www.quora.com/What-is-the-difference-between-a-message-queue-and-a-task-queue-Why-would-a-task-queue-require-a-message-broker-like-RabbitMQ-Redis-Celery-or-IronMQ-to-function)
+
+## Communication
+
+<p align="center">
+  <img src="http://i.imgur.com/5KeocQs.jpg">
+  <br/>
+  <i><a href=http://www.escotal.com/osilayer.html>Source: OSI 7 layer model</a></i>
+</p>