Add Changelog

Please add a changelog.md according to https://keepachangelog.com/en/1.1.0/.

Please init the changelog with a rough summary of the releases that happened as well as the changes that are upcoming (in the [Unreleased] part).

Note that a CHANGELOG will never be perfect so don't go into too much detail but make sure you cover the most important parts in an understandable manner. The CHANGELOG is meant for users of your code i.e. it should concentrate on changes that actually affect the API or UI (no refactoring if it was not major). The goal is to have a single go-to documentation that we can a) copy to the release notes and b) communicate with other team members (mainly non programmers), partners and outreach.

See https://gitlab.gistools.geog.uni-heidelberg.de/climate-action/climatoology/-/blob/main/CHANGELOG.md for an example.