Add changelog for the first release

CHANGELOG.md

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+# Changelog
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
+and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
+
+## 1.0.0 (2020-07-08)
+First release of this package; features include:
+ - A Symfony Messenger transport that relies on MongoDB, using [`facile-it/mongodb-bundle`](https://github.com/facile-it/mongodb-bundle/)
+ - The bundle support to be used in a Symfony app
+ - An extension point to enrich the persisted document, using the `DocumentEnhancer` interface
+ - A non-sendable `ReceivedStamp` stamp class dedicated to obtain the ID of the persisted document
+ - A `RedeliveryStampExtractor` utility class