add hello world and app intro

meh not a fan of what im writing today but gotta get this thing out

docs/api/index.md

@@ -1,8 +1,28 @@
 # Application
 
-  A Koa application is not a 1-to-1 representation of an HTTP server,
-  as one or more Koa applications may be mounted together to form larger
-  applications, with a single HTTP server.
+  A Koa application is an object containing an array of middleware generator functions
+  which are composed and executed in a stack-like manner upon request. Koa is similar to many
+  other middleware systems that you may have encountered such as Ruby's Rack, Connect, and so on -
+  however a key design decision was made to provide high level "sugar" at the otherwise low-level
+  middleware layer. This improves interoperability, robustness, and makes writing middleware much
+  more enjoyable.
+
+  This includes methods for common tasks like content-negotation, cache freshness, proxy support, and redirection
+  among others. Despite supplying a reasonably large number of helpful methods Koa maintains a small footprint, as
+  no middleware are bundled.
+
+  The obligatory hello world application:
+
+```js
+var koa = require('koa');
+var app = koa();
+
+app.use(function *(){
+  this.body = 'Hello World';
+});
+
+app.listen(3000);
+```
 
 ## Settings
 
@@ -18,6 +38,10 @@
 
 ## app.listen(...)
 
+  A Koa application is not a 1-to-1 representation of an HTTP server,
+  as one or more Koa applications may be mounted together to form larger
+  applications, with a single HTTP server.
+
   Create and return an HTTP server, passing the given arguments to
   `Server#listen()`. These arguments are documented on [nodejs.org](http://nodejs.org/api/http.html#http_server_listen_port_hostname_backlog_callback). The following is a useless Koa application bound to port `3000`: