Add codecov badge to README.md

[nstarman]

Just noticed in one of my projects that there isn't a badge for code coverage. I can open a PR if you're interested.

[phinate]

is there a way to reliably pre-produce a working codecov link for a project for the badge?

[nstarman]

I believe so:
https://codecov.io/<codehost>/<orgname>/<reponame>/\branch/<branchname>/\graph/\badge.svg?

(see https://docs.codecov.com/docs/status-badges)

[henryiii]

I think that would make sense. Long term, I'd like some sort of "checkbox" feature where things could be selected or deselected, then codecov could be an option. But I think we are somewhat suggesting codecov at the moment. So a badge makes sense. It could be removed more easily than added.