Add CHANGELOG

Author: motdotla

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file. See [standard-version](https://github.com/conventional-changelog/standard-version) for commit guidelines.
+
+## [Unreleased](https://github.com/dotenv-org/dotenv-vault-python/compare/v0.0.9...master)
+
+## 0.0.9
+
+### Added
+
+- Decrypting .env.vault file when `DOTENV_KEY` is set.
+
+## 0.0.8 and prior
+
+Please see commit history.