Adds CHANGELOG entry for #534

Author: weierophinney

CHANGELOG.md

@@ -30,6 +30,13 @@ All notable changes to this project will be documented in this file, in reverse
   requires it. This addition fixes problems due to missing http-middleware
   interfaces.
 
+- [#534](https://github.com/zendframework/zend-expressive/pull/534) provides a
+  fix for how it detects `callable` middleware. Previously, it relied on PHP's
+  `is_callable()`, but that function can result in false positives when provided
+  a 2-element array where the first element is an object, as the function does
+  not verify that the second argument is a valid method of the first. We now
+  implement additional verifications to prevent such false positives.
+
 ## 2.0.4 - 2017-10-09
 
 ### Added