Added "Keep a changelog"

Author: linkdotnet

CHANGELOG.md

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+# Changelog
+
+All notable changes to **ValueStringBuilder** will be documented in this file. The project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+<!-- The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/) -->
+
+## [Unreleased]
+
+This is the initial release for the `StringBuilder`.
+
+### Added
+ - `StringBuilder` with some major methods