Jan Hereš

Adding a CHANGELOG for us to be able to effectively generate release notes. The format should abide to [https://keepachangelog.com/en/1.1.0/](https://keepachangelog.com/en/1.1.0/)

### Checklist - [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues). - [X] For Python issues, I have tested with the [latest development wheel](https://www.open3d.org/docs/latest/getting_started.html#development-version-pip). - [X] I have checked the [release...