basic readme explanations

follow-up to #160

- [ ] add a bullet list of files "of highest public interest" from https://github.com/iterative/cml-playground/tree/main/.github/workflows described, e.g.:

```md
- Auto-detect long-running (potentially [orphaned](https://github.com/iterative/cml/issues/1014))
[`cml runner launch --cloud=aws`](https://cml.dev/doc/self-hosted-runners?tab=AWS#options)
instances & create GitHub issues:
[checks-lights-aws](https://github.com/iterative/cml-playground/blob/main/.github/workflows/checks-lights-aws.yml)
```

- [ ] also can we always either dogfood or add detailed comments EVERYWHERE as to why we're not dogfooding? e.g.:

```diff
+# not using `cml comment create ./report.md`
+# because this workflow is intended to test if `cml` is broken
echo "::set-output name=report::$(cat ./report.md)"
```