Document that a Changelog will not be kept in a file, but for the future that the Github Releases feature will be used

Author: fbartho

CHANGELOG.md

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+# Changelog
+
+This project hasn't kept a changelog historically, and it would be infeasible to retroactively create one.
+
+Instead, for the near future, releases will be encapsulated by a [milestone](https://github.com/taskrabbit/react-native-zendesk-chat/milestones) and [Github Releases](https://github.com/taskrabbit/react-native-zendesk-chat/releases)