Add reference to hacking docs to README.

README.md

 * There is also a [Heroku buildpack](https://github.com/jsmpereira/coleslaw-heroku) maintained by Jose Pereira.
 * Example sites: [redlinernotes](http://redlinernotes.com/blog/), [kenan-bolukbasi.log](http://kenanb.com/), and [Nothing Really Matters](http://ironhead.xs4all.nl/).
 
+## Hacking
+
+A core goal of *coleslaw* is to be both pleasant to read and easy to hack on and extend. If you want to understand the internals and bend *coleslaw* to do new and interesting things, I strongly encourage you to read the [Hacker's Guide to Coleslaw](https://github.com/redline6561/coleslaw/blob/master/docs/hacking.md).
+
 ## Installation
 This software should be portable to any conforming Common Lisp implementation but this guide will assume SBCL is installed. Testing has also been done on CCL.
 Server side setup: