adds a changelog

[johannesjh]

Adds a changelog. refs #53

• For the changelog, I simply added a new .rst file as part of req2flatpak's documentation.
• I wrote the file by hand because this is simpler than using tools. And changelog contents should be hand-crafted in my opinion, to make them useful for users.
• I added instructions in docs/source/development.rst so we don't forget to update the changelog when creating a release.

[cbm755]

I agree about hand-crafting. Here's a reference I found useful: https://keepachangelog.com

[cbm755]

LGTM!

[real-yfprojects]

LGTM. Do we want to append the changelog to the pypi page?

[johannesjh]

Do we want to append the changelog to the pypi page?

could be useful... I added changelog.rst to the list of readme file in pyproject.toml.