Add changelog information to docs

[connortann]

Many repos keep a changelog. Recommended practices / style are here: https://keepachangelog.com/en/1.0.0/

Suggested actions:

• Add CHANGELOG.md to repo, in format detailed above
• Key changes of past few releases populated
• Include changelog into build documentation (e.g. markdown-to-RST extension here)

[connortann]

Maybe we could simply point people to the releases page?

https://github.com/bp/resqpy/releases

[connortann]

A better solution would be to include the release information directly within the documentation. This seems doable:

1. Could generate markdown text from GitHub Releases API, using this plugin:
   https://github.com/rhysd/changelog-from-release

2. Could use this action to allow uploading of built artifacts into the ReadTheDocs process:
   https://github.com/dfm/rtds-action

3. Enable parsing of Markdown files in Sphinx:
   https://www.sphinx-doc.org/en/1.5.3/markdown.html