add debugging example

debugging/index.js

@@ -0,0 +1,37 @@
+
+/**
+ * Module dependencies.
+ */
+
+var koa = require('koa');
+var app = koa();
+
+// to enable debugging output per middleware
+// run with the following environment variable:
+// $ DEBUG=koa-compose node --harmony debugging
+
+// note how the function names are used
+// in the debug output.
+
+app.use(function *json(next){
+  yield next;
+  this.body = { message: this.body };
+});
+
+app.use(function *reverse(next){
+  yield next;
+  this.set('X-Bar', 'baz');
+  this.body = this.body.split('').reverse().join('');
+});
+
+app.use(function *uppercase(next){
+  yield next;
+  this.set('X-Foo', 'bar');
+  this.body = this.body.toUpperCase();
+});
+
+app.use(function *response(next){
+  this.body = 'Hello World'
+});
+
+app.listen(3000);