Adds CHANGELOG entry for #313

Author: weierophinney

CHANGELOG.md

@@ -10,7 +10,8 @@ All notable changes to this project will be documented in this file, in reverse
 
 ### Changed
 
-- Nothing.
+- [#313](https://github.com/zendframework/zend-diactoros/pull/313) changes the reason phrase associated with the status code 425
+  to "Too Early", corresponding to a new definition of the code as specified by the IANA.
 
 ### Deprecated