Provide API docs for createProvider

[dashed]

I'm unsure if it's intentionally not documented.

I had to dive into the source code to see how it works 😄

[markerikson]

Y'know, that's a good point - we added it, and we're exposing it out of index.js so it's part of the public API, but we haven't documented it.

If you could submit a PR with docs for createProvider, we can definitely merge that in.