docs: add CHANGELOG

CHANGELOG.md

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+<!--
+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/).
+-->
+
+## Version 2.0.1
+
+### Bug fixes
+
+#### x/marketmap
+
+* Fixed the default genesis state generation ([89f6385](https://github.com/milkyway-labs/milkyway/commit/89f638567af91e819e6ae3948823b55a24292d61))
+
+## Version 2.0.0
+
+This is the first release of the new major version of the project.
+The main change that has been made is the transition from being an L2 Optimistic Rollup to being an L1 Cosmos-SDK based
+chain.
+
+Aside from this, various bugs have been fixed and useful features has been implemented. You can see the full list of
+changes [here](https://github.com/milkyway-labs/milkyway/compare/v1.6.0...v2.0.0).