Mention supervisor setup in documentation and blog post

[jaimeiniesta]

Hi!

I'm learning to use Haystack, and found that the documentation and blog post are missing an important step, where you set up the related supervisor and storage:

https://github.com/elixir-haystack/phoenix-live-view-example/blob/main/lib/app/application.ex#L20-L21

[philipbrown]

Thank you @jaimeiniesta!

Yeah, I still need to go expand on the documentation. There's a couple of places that are still lacking. Let me know if you find anything else that requires more documentation!