add DEBUG usage docs. Closes #41

docs/guide.md

@@ -103,8 +103,7 @@ app.use(logger(':method :url'));
 
 ### Named middleware
 
-  Naming middleware is optional, however it's useful for debugging purposes to
-  assign a name.
+  Naming middleware is optional, however it's useful for debugging purposes to assign a name.
 
 ```js
 function logger(format){
@@ -136,3 +135,20 @@ app.use(function *(){
 });
 ```
 
+## Debugging Koa
+
+  Koa along with many of the libraries it's built with support the __DEBUG__ environment variable from [debug](https://github.com/visionmedia/debug) which provides simple conditional logging.
+
+  For example
+  to see all koa-specific debugging information just pass `DEBUG=koa*` and upon boot you'll see the list of middleware used, among other things.
+
+```
+$ DEBUG=koa* node --harmony examples/simple
+  koa:application use responseTime +0ms
+  koa:application use logger +4ms
+  koa:application use contentLength +0ms
+  koa:application use notfound +0ms
+  koa:application use response +0ms
+  koa:application listen +0ms
+```
+