chore: add changelog header for 2.5.0.0.0

Author: kantai

CHANGELOG.md

@@ -5,7 +5,16 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to the versioning scheme outlined in the [README.md](README.md).
 
-## [Next-Branch]
+## [2.5.0.0.0]
+
+This release implements the 2.5 Stacks consensus rules: primarily the instantiation
+of the pox-4 contract. For more details see SIP-021.
+
+This release is a consensus-critical release. Nodes which do not update before
+the 2.5 activation height will be forked away from the rest of the network. This release
+is compatible with 2.4.x chain state directories and does not require resyncing from
+genesis. The first time a node boots with this version it will perform some database
+migrations which could lengthen the normal node startup time.
 
 ### Added