Add changelog

changelog.md

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+# Change Log
+
+This file contains all the notable changes done to the Ballerina Copybook tool through the releases.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/), and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+- [[#7271] Add Support for Implied Decimal](https://github.com/ballerina-platform/ballerina-library/issues/7271)