Add Asynchronism intro section

README.md

@@ -1397,3 +1397,13 @@ Refresh-ahead can result in reduced latency vs read-through if the cache can acc
 * [Scalability](http://www.lecloud.net/post/9246290032/scalability-for-dummies-part-3-cache)
 * [AWS ElastiCache strategies](http://docs.aws.amazon.com/AmazonElastiCache/latest/UserGuide/Strategies.html)
 * [Wikipedia](https://en.wikipedia.org/wiki/Cache_(computing))
+
+## Asynchronism
+
+<p align="center">
+  <img src="http://i.imgur.com/54GYsSx.png">
+  <br/>
+  <i><a href=http://lethain.com/introduction-to-architecting-systems-for-scale/#platform_layer>Source: Intro to architecting systems for scale</a></i>
+</p>
+
+Asynchronous workflows help reduce request times for expensive operations that would otherwise be performed in-line.  They can also help by doing time-consuming work in advance, such as periodic aggregation of data.