Expand config docs

[jefftriplett]

Feel free to close this, but I struggled to figure out how to configure this plugin, what the config file should be called, and where it should live.

I skimmed docs and started with config.json, but I keep seeing The path plugin configuration is required.

Eventually, I noticed datasette serve --metadata=metadata.json in some examples from your main docs. (I'll submit a patch for a typo I noticed too.)

I'm opening this for anyone else who tries it out and runs into this issue.

[simonw]

Yeah, this is a wider problem with Datasette: metadata.json started out as a feature for adding license/source metadata, but it grew all kinds of extra features related to configuration, especially for plugins.

I have an open issue for fixing that here: simonw/datasette#493 - it's included in the 1.0 milestone, but it's going to take a fair bit of work to redesign everything.

In the meantime, expanding the documentation for this plugin feels like a good idea.