update changelog

Author: djih

CHANGELOG.md

@@ -1,5 +1,7 @@
 ## Unreleased
 
+### 2.10.0 (March 30, 2016)
+
 * Identify function now accepts a callback function as an argument. The callback will be run after the identify event is sent.
 * Add support for `prepend` user property operation. This allows you to insert value(s) at the front of a list. See [Readme](https://github.com/amplitude/Amplitude-Javascript#user-properties-and-user-property-operations) for more details.
 * Keep sessions and event metadata in-sync across multiple windows/tabs.