docs: Add more badges

[eliotwrobson]

Added a dynamic badge to display supported Python versions and a docs building badge (since this was something requested as part of the ongoing review): pyOpenSci/software-submission#152 (comment)

[coveralls]

coverage: 99.612%. remained the same
when pulling b992e4c on eliotwrobson:badge
into bbf3d58 on caleb531:main.

[caleb531]

This looks good! Just please remember to rebase develop off of the latest main after merging.

git checkout main
git pull
git checkout develop
git rebase -i main
git push