add an example to facebook readme file

docs/readme-facebook.md

@@ -20,6 +20,7 @@ Table of Contents
 * [Simulate typing](#simulate-typing)
 * [Silent and No Notifications](#silent-and-no-notifications)
 * [Messenger code API](#messenger-code-api)
+* [Attachment upload API](#attachment-upload-api)
 * [Running Botkit with an Express server](#use-botkit-for-facebook-messenger-with-an-express-web-server)
 
 ## Getting Started
@@ -506,6 +507,38 @@ controller.api.messenger_profile.get_target_audience(function (err, data)  {
 
 ```
 
+## Attachment upload API
+
+Attachment upload API allows you to upload an attachment that you may later send out to many users, without having to repeatedly upload the same data each time it is sent :
+
+
+```js
+var attachment = {
+        "type":"image",
+        "payload":{
+            "url":"https://pbs.twimg.com/profile_images/803642201653858305/IAW1DBPw_400x400.png",
+            "is_reusable": true
+        }
+    };
+
+    controller.api.attachment_upload.upload(attachment, function (err, attachmentId) {
+        if(err) {
+            // Error
+        } else {
+            var image = {
+                "attachment":{
+                    "type":"image",
+                    "payload": {
+                        "attachment_id": attachmentId
+                    }
+                }
+            };
+            bot.reply(message, image);
+        }
+    });
+
+```
+
 
 ## Use BotKit for Facebook Messenger with an Express web server
 Instead of the web server generated with setupWebserver(), it is possible to use a different web server to receive webhooks, as well as serving web pages.