Update schema.json and README

[steven11sjf]

The README currently says

The API/Schema for the input json file will soon be documented.

Cross-referencing the schema.json and SeedObject it seems like the schema partially documents it but is missing some fields (at first glance, starting_items and pickups are missing documentation and pipes is not a field). This should be updated to be consistent, and the README should direct users to schema.json.

It looks like you could auto-generate a schema.json based on the SeedObject, based on this StackOverflow post. I'd also look into adding CI to ensure schema matches and see if there's a way to add property attributes for schema fields like description.