Adds provider, model vars docs to README

[falquaddoomi]

The upcoming rootstock PR manubot/rootstock#522 adds a few new optional repo variables and secrets, specifically:

• AI_EDITOR_MODEL_PROVIDER which specifies a default model provider when "(repo default)" is selected in the workflow model provider dropdown, and
• AI_EDITOR_LANGUAGE_MODEL which specifies a default model to use

The new rootstock PR allows provider-specific API keys in addition to the generic PROVIDER_API_KEY already mentioned in the docs.

This PR adds the two variables mentioned above plus these new API key secrets to the README section regarding setting up a rootstock fork and running the ai-editor workflow.

[d33bs]

I could be imagining it but was there a reference to an environment variable glossary somewhere? Consider adding a link somewhere nearby if so.

[falquaddoomi]

There is; it's further down in the document in the "Using from the command line" subsection. Good idea to add a link to it here, too, though, since it elaborates on how API keys are resolved.

[falquaddoomi]

FYI, the link has been added as of 4e5c235.

[miltondp]

Thanks, @falquaddoomi, for this PR and @d33bs for the review. If you don't mind, I will take a look at the new release after this is merged, so I can test it with manuscripts we have. Let me know, though, if you need me to look at this PR at this stage.