Readme is subtly out of date regarding workflows and API keys

[EliahKagan]

This still gives the right idea, but it is out of date:

EmbeddingScratchwork/README.md

Lines 251 to 253 in caada01

	Once enabled, some of the workflows will run without problems. Some others—the
	automated tests in [`test_embed`](tests/test_embed.py)—cannot run successfully
	without an OpenAI API key.

There are two problems:

• We have other test modules than test_embed.py, some of which have tests that require OPENAI_API_KEY.
• No test modules run on CI without that secret set up, because the workflow is written to check for it first.

So this should be updated. But we should avoid inadvertently claiming that every test case requires OPENAI_API_KEY, which isn't the case: there are some tests that pass without it (and should), just we're not running any tests on CI when OPENAI_API_KEY is unavailable. Preferably the explanation should be kept as short and simple as it is now, though.