docs: add setup config and backends to the readme

Summary:

Now we have a setup function with some config we better tell ppl about it. This
add the setup and config options into the readme.

It also replaces the "Command" with "Backends" to make it a little easier to
understand what needs to be configured. The backend concept has only just been
exposed publicly recently. It now makes since rename this section to line up
with the config.

Test Plan:

N / A