Documentation: Add badges (e.g., code coverage) to the top-level README

Split off from #34

Currently in the middle of fleshing out our tests, but once we have that in place, I think that we should be upfront as we can about the status of the project. Conveying things like code coverage and builds passing would go a long way towards giving people confidence in our offerings

Start with the top-level README and inspect the repository's existing test and build configuration to identify the status information available for badges. The work is done when the README shows relevant coverage and build-status badges that accurately reflect the project's current checks.

索引モデルが issue の本文から書いたものです。