Badge/indicator of GHA storage use

[mattwthompson]

Some automation (namely, the constructor in the toolkit) can generate large-ish artifacts on the order of 100s of MB. So we don't accidentally use our free storage, it would be nice to track on the README how much is being used.

It looks like it's free in some capacity for public repos, although I can't tell if there's a soft limit. So, if/until we run into issues, I don't think this is something to worry about.

GitHub Actions usage is free for public repositories and self-hosted runners.

https://docs.github.com/en/free-pro-team@latest/github/setting-up-and-managing-billing-and-payments-on-github/about-billing-for-github-actions#about-billing-for-github-actions

There is setting for how many days the logs and actions persist (here). It appears to have defaulted to 90, and I just changed it to its max of 400.