Update CHANGELOG.md

Co-authored-by: Chris Brozdowski <CBrozdowski@yahoo.com>

Author: Thinh Nguyen

CHANGELOG.md

@@ -3,7 +3,7 @@
 Observes [Semantic Versioning](https://semver.org/spec/v2.0.0.html) standard and
  [Keep a Changelog](https://keepachangelog.com/en/1.0.0/) convention.
 
-## [Unreleased]
+## [0.2.1] - Unreleased
 
 + Update - clustering step, update duration for "median_subtraction" step
 + Bugfix - handles single probe recording in "Neuropix-PXI" format