Mention X-Callback-Url

guide/chaining_functions.md

@@ -79,3 +79,15 @@ GitHub sends a "star" event to tweetfanclub function, tweetfanclub uses get-avat
 This example uses a mix of regular binaries such as ImageMagick and Python handlers generated with the FaaS-CLI.
 
 * [GitHub to Twitter Fanclub](https://github.com/alexellis/faas-twitter-fanclub/blob/master/README.md)
+
+## Asynchronous call-backs
+
+If you invoke a function asynchronously you have two options for getting the result back:
+
+* Update the function
+
+You can update your code to call another function / store state in another service
+
+* X-Callback-Url
+
+If you set a header for `X-Callback-Url` then that will be invoked after the function has run, [read more](https://github.com/openfaas/faas/blob/1aa6270fcc274cc36d90e0a9e4caa3eb71912ae0/guide/asynchronous.md#call-a-function)