Add an example of onresponse hooks

Also fix the guide entry on hooks.

examples/error_hook/README.md

@@ -0,0 +1,30 @@
+Cowboy Error Hook
+=================
+
+To compile this example you need rebar in your PATH.
+
+Type the following command:
+```
+$ rebar get-deps compile
+```
+
+You can then start the Erlang node with the following command:
+```
+./start.sh
+```
+
+Then point your browser to the indicated URL.
+
+Example
+-------
+
+``` bash
+$ curl -i http://localhost:8080
+HTTP/1.1 404 Not Found
+connection: keep-alive
+server: Cowboy
+date: Wed, 27 Feb 2013 23:32:55 GMT
+content-length: 56
+
+404 Not Found: "/" is not the path you are looking for.
+```