Create a specific CHANGELOG.md file and set changelog_uri

[RiwanTalmo]

Hey 👋

Thank you very much for your gem.
I realized that the changelog was located at the very bottom of the readme. Therefore, I would like to propose a few changes:

• Creation of a dedicated CHANGELOG.md file to follow best practices
• Definition of the changelog_uri so the changelog can be found more easily, for example by GemUpdater.

Thank you!