expand README

linked to writing unit generators helpfile and the SC book, and added a directory listing

Author: Nathan Ho

README.md

@@ -1,7 +1,14 @@
-This repository a series of examples demonstrating how to write server plugins for [SuperCollider](https://github.com/supercollider/supercollider) (not to be confused with quarks, which are libraries for the language).
+This repository a series of examples demonstrating how to write UGens for [SuperCollider](https://github.com/supercollider/supercollider) (not to be confused with quarks, which are libraries for the language). The [Writing Unit Generators](http://doc.sccode.org/Guides/WritingUGens.html) helpfile is a comprehensive tutorial. Chapter 25 of [the SuperCollider Book](http://www.supercolliderbook.net/) is also a useful resource, although the build instructions are outdated.
 
 Beyond this repository, the reader is encouraged to look at [sc3-plugins](https://github.com/supercollider/sc3-plugins) for more complex, real-world examples.
 
+## Directory
+
+- 01a-BoringMixer -- minimal example of a plugin
+- 01b-BoringMixer -- using a newer C++ wrapper
+- 02-MySaw -- introduces multiple calculation functions and state variables
+- 03-AnalogEcho -- introduces memory allocation and cubic interpolation
+
 ## Compiling
 
 Make a directory for the `cmake` build files: