Document accessing response body in after filter. Fixes #168.

README.rdoc

@@ -699,6 +699,10 @@ and routes are accessible by after filters:
     puts response.status
   end
 
+Note: Unless you use the `body` method rather than just returning a String from
+the routes, the body will not yet be available in the after filter, since it is
+generated later on.
+
 Filters optionally taking a pattern, causing them to be evaluated only if the
 request path matches that pattern: