Add Cache intro section

Author: donnemartin

README.md

@@ -1206,3 +1206,15 @@ Sample data well-suited for NoSQL:
 
 * [Scaling up to your first 10 million users](https://www.youtube.com/watch?v=vg5onp8TU6Q)
 * [SQL vs NoSQL differences](https://www.sitepoint.com/sql-vs-nosql-differences/)
+
+## Cache
+
+<p align="center">
+  <img src="http://i.imgur.com/Q6z24La.png">
+  <br/>
+  <i><a href=http://horicky.blogspot.com/2010/10/scalable-system-design-patterns.html>Source: Scalable system design patterns</a></i>
+</p>
+
+Caching improves page load times and can reduce the load on your servers and databases.  In this model, the dispatcher will first lookup if the request has been made before and try to find the previous result to return, in order to save the actual execution.
+
+Databases often benefit from a uniform distribution of reads and writes across its partitions.  Popular items can skew the distribution, causing bottlenecks.  Putting a cache in front of a database can help absorb uneven loads and spikes in traffic.