Add Changelog.

Author: Maikuolan

Changelog.md

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+### v3.0.0
+
+[2020.07.20; Maikuolan] First stable release for the phpMussel v3 Core.
+
+__*Why "v3.0.0" instead of "v1.0.0?"*__ Prior to phpMussel v3, the "phpMussel Core", "phpMussel CLI-mode", "phpMussel Front-End", and "phpMussel Uploads Handler" ("phpMussel Web") were all bundled together as a single repository (phpMussel/phpMussel). Since phpMussel v3, these each all have their own, separate repositories. I've opted to start releases at this repository (phpMussel/Core) at *v3.0.0*, in order to avoid confusion with previous versions of the "phpMussel Core" which exist outside this repository.