Document code coverage report

[iandunn]

I tried to find the report by guessing what the coveralls.io URL would be, but wasn't successful. Adding it to the readme could be helpful. I'm happy to open a PR if someone can share the URL.

[kasparsd]

After being moved to the WordPress org the URL is https://coveralls.io/github/WordPress/two-factor but it used to be under @georgestephanis vendor https://coveralls.io/github/georgestephanis/two-factor

I haven't checked if the new one is even configured.

[iandunn]

Thanks! It doesn't seem to be showing any reports, so I guess documenting should probably wait until it's configured 👍🏻

Related #468

[iandunn]

Related #364