docs: revise README for repository migration details (#1133)

Updated README to reflect repository migration to Supabase JS Monorepo.

## What kind of change does this PR introduce?

Bug fix, feature, docs update, ...

## What is the current behavior?

Please link any relevant issues here.

## What is the new behavior?

Feel free to include screenshots if it includes visual changes.

## Additional context

Add any other context or screenshots.