Add NEWS/Changelog file to summarise key changes between versions

[gregorgorjanc]

We should add a NEWS/Changelog file to summarise key changes between versions and each PR addressing an issue should add a line or two so we grow this file “organically” and then have it ready for new version release.

@XingerTang and @AprilYUZhang what is the Python way of doing this?

[AprilYUZhang]

maybe slap can help us, but I haven't use it.

[XingerTang]

@gregorgorjanc I haven't used Python-specific changelog files before. Maybe we can add a rst file for it, this is how the read the docs write their change logs, so it can be easily integrated into the documentation.

[gregorgorjanc]

RST or MD would work just well https://github.com/tskit-dev/msprime/blob/main/CHANGELOG.md

[XingerTang]

https://github.com/vweevers/common-changelog#12-guiding-principles gives a style guide for changelogs, and https://alphaos.app/blog/github-changelog-tools gives some Github changelog generators