WHY
Some customers use the upstream docs and it's important that we have an accurate and up-to-date changelog.

WHAT
Update 3scale-operator's CHANGELOG.md to reflect all of the changes since it was last updated.

It would also be great to set up an automated Git tool that continuously updates the changelog as PRs are merged. If it's not feasible to use an automated tool then we should at least have a document that describes how we plan to manage the changelog in the future.

DONE

• The changelog is up to date with the latest changes
• If possible, set up an automated tool to continuously update the changelog going forward