Added CHANGELOG for 3.1.0

Author: weierophinney

CHANGELOG.md

@@ -2,31 +2,22 @@
 
 All notable changes to this project will be documented in this file, in reverse chronological order by release.
 
-## 3.1.0 - TBD
-
-### Added
-
-- Nothing.
-
-### Deprecated
-
-- Nothing.
-
-### Removed
-
-- Nothing.
-
-### Fixed
-
-- Nothing.
-
-## 3.0.2 - TBD
+## 3.1.0 - 2016-12-19
 
 ### Added
 
 - [#26](https://github.com/zendframework/zend-eventmanager/pull/26) publishes
   the documentation to https://zendframework.github.io/zend-eventmanager/
 
+### Changes
+
+- [#17](https://github.com/zendframework/zend-eventmanager/pull/17) makes a
+  number of internal changes to how listeners are stored in order to improve
+  performance, by as much as 10% in the scenario used in the MVC layer.
+  
+  Additionally, it optimizes when the target and event arguments are injected
+  into an event, eliminating that step entirely when either is unavailable.
+
 ### Deprecated
 
 - Nothing.