add example for m.request config

docs/mithril.request.md

@@ -226,6 +226,20 @@ var file = m.request({
 });
 ```
 
+### Configuring the underlying XMLHttpRequest
+
+The `config` option can be used to arbitrarily configure the native XMLHttpRequest instance and to access properties that would not be accessible otherwise.
+
+The example below show how to configure a request where the server expects requests to have a `Content-Type: application/json` header
+
+```javascript
+var xhrConfig = function(xhr) {
+	xhr.setRequestHeader("Content-Type", "application/json");
+}
+
+m.request({method: "POST", url: "/foo", config: xhrConfig});
+```
+
 ---
 
 ### Signature