Add additional files

[lilyminium]

This PR adds the files we are accustomed to in https://github.com/MDAnalysis/mdanalysis but are not in MolSSI's cookiecutter. However, I have made the following changes:

• they're all Markdown
• The "rules" are commented so only visible to people editing the file
• I adapted the "Keep a Changelog" sections and made the contributors an explicit section, instead of hewing more closely to MDAnalysis's current format

My reasoning for this is that Markdown is just much easier to work with than rst; you can preview it and type directly into GitHub, for example. Having some kind of formatting also makes it easier to read in GitHub instead of just a straight text file. To get authors for the documentation might require a little more fancy regex, but not by much.

Please see examples at:

• https://github.com/lilyminium/test_mda_condaforge_rtd/blob/main/AUTHORS.md
• https://github.com/lilyminium/test_mda_condaforge_rtd/blob/main/CHANGELOG.md
• https://github.com/lilyminium/test_mda_condaforge_rtd/blob/main/CONTRIBUTING.md

I'll do code of conduct in a separate one.

[lilyminium]

Oops, sorry, wrong destination!