Add the ability for crate authors to add code coverage badges to their crate #706
Description
Part of RFC 1824, rust-lang/rust#41616.
Start with Coveralls and Codecov.
We will enable maintainers to add these badges to their crate, in a similar way that the TravisCI/Appveyor/Gitlab badges are implemented today.
Also need to update the documentation that currently lives in cargo's repo.
Please let me know if you have any questions, potential implementers!