Update README.md

explain how to add xhr option

README.md

@@ -298,6 +298,24 @@ request.errback { |response|
 }
 ```
 
+### Supplying an XHR method
+
+To supply an XHR callback include a lambda with the `xhr` option:
+
+```ruby
+update_progress = lambda do
+  xhr = `new window.XMLHttpRequest()`
+  update_progress = lambda do |evt|
+    # update your progress here
+  end
+  `xhr.upload.addEventListener("progress", update_progress, false)`
+  xhr
+end
+
+cloud_xfer = HTTP.put "http://my.cloud.storage/location", xhr: update_progress, ... etc ...
+```
+
+
 ## Usage of JQuery plugins
 Extra plugins used for JQuery aren't available to ruby code by default, you will have to `expose` these functions to opal-jquery.
 
@@ -314,6 +332,7 @@ el = Element['.html_element']
 el.cool_plugin({argument: 'value', argument1: 1000}.to_n)
 ```
 
+
 ##  License
 
 (The MIT License)