Adds CHANGELOG entry for #320

Author: weierophinney

CHANGELOG.md

@@ -22,6 +22,10 @@ All notable changes to this project will be documented in this file, in reverse
 
 ### Fixed
 
+- [#320](https://github.com/zendframework/zend-diactoros/pull/320) adds checking within `Response` to ensure that the provided reason
+  phrase is a string; an `InvalidArgumentException` is now raised if it is not. This change
+  ensures the class adheres strictly to the PSR-7 specification.
+
 - [#319](https://github.com/zendframework/zend-diactoros/pull/319) provides a fix to `Zend\Diactoros\Response` that ensures that the status
   code returned is _always_ an integer (and never a string containing an
   integer), thus ensuring it strictly adheres to the PSR-7 specification.