docs: add context example

docs/api/context.md

@@ -12,7 +12,16 @@
   which would force middlware to re-implement this common functionality.
 
   A `Context` is created _per_ request, and is referenced in middleware
-  as the receiver, or the `this` identifier.
+  as the receiver, or the `this` identifier, as shown in the following
+  snippet:
+
+```js
+app.use(function *(){
+  this; // is the Context
+  this.request; // is a koa Request
+  this.response; // is a koa Response
+});
+```
 
 ## Request aliases