Create CHANGELOG.md

[jank324]

We should create and maintain a CHANGELOG.md.

I would propose a format similar to the following (mix of Stable-Baselines3 and https://keepachangelog.com/en/1.0.0/).

E.g.

Unreleased

🚀 Features

• Better error messages when beam and accelerator devices don't match (see#42) (@maxmustermann)

💥 Breaking

• None

🐛 Bug fixes

• Fix BPM issue with ParameterBeam (see#42) (@maxmustermann)
• Fix wrong screen reading dimensions (see#42) (@maxmustermann)

🐆 Other

• Improve docstrings (see#42) (@maxmustermann)
• Implement better testing with pytest (see#42) (@maxmustermann)
• Setup formatting with black and isort as well as flake8 listing (see#42) (@maxmustermann)
• Add type annotations (see#42) (@maxmustermann)
• Setup Sphinx documentation on readthedocs (see#42) (@maxmustermann)

v0.3

🚀 Features

• Better error messages when beam and accelerator devices don't match (see#42) (@maxmustermann)

💥 Breaking

• None

🐛 Bug fixes

• Fix BPM issue with ParameterBeam (see#42) (@maxmustermann)
• Fix wrong screen reading dimensions (see#42) (@maxmustermann)

🐆 Other

• Improve docstrings (see#42) (@maxmustermann)
• Implement better testing with pytest (see#42) (@maxmustermann)
• Setup formatting with black and isort as well as flake8 listing (see#42) (@maxmustermann)
• Add type annotations (see#42) (@maxmustermann)
• Setup Sphinx documentation on readthedocs (see#42) (@maxmustermann)