Adds CHANGELOG entry for #285

Author: weierophinney

CHANGELOG.md

@@ -6,7 +6,11 @@ All notable changes to this project will be documented in this file, in reverse
 
 ### Added
 
-- Nothing.
+- [#285](https://github.com/zendframework/zend-diactoros/pull/285) adds a new
+  custom response type, `Zend\Diactoros\Response\XmlResponse`, for generating
+  responses representing XML. Usage is the same as with the `HtmlResponse` or
+  `TextResponse`; the response generated will have a `Content-Type:
+  application/xml` header by default.
 
 ### Changed