Add the WebWorker API docs

The Readme says

If you don't want to run CPU-intensive SQL queries in your main application thread, you can use the more limited WebWorker API.

What exactly it does mean? What limitations the WebWorker API have? And where I can find the WebWorker API docs?

Start with the README passage about the WebWorker API and inspect the project's existing WebWorker API entry points or documentation. Document what the API does, explain its limitations compared with the main application thread, and provide a link or location for the API docs.