add release date to changelog

Author: frangio

packages/cli/changelog.md

@@ -4,7 +4,7 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
-## v2.8.0 - (unreleased)
+## v2.8.0 - 2020-03-25
 
 ### Added
 - Added new `deploy` command for deploying without upgradeability. ([#1376](https://github.com/OpenZeppelin/openzeppelin-sdk/pull/1376))