AutoWorld: add preliminary .apworld specification (ArchipelagoMW#903)

* AutoWorld: add preliminary .apworld specification

* Doc: apworld specification: fix typo

docs/apworld specification.md

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+# apworld Specification
+
+Archipelago depends on worlds to provide game-specific details like items, locations and output generation.
+Those are located in the `worlds/` folder (source) or  `<insall dir>/lib/worlds/` (when installed).
+See [world api.md](world api.md) for details.
+
+apworld provides a way to package and ship a world that is not part of the main distribution by placing a `*.apworld`
+file into the worlds folder.
+
+
+## File Format
+
+apworld files are zip archives with the case-sensitive file ending `.apworld`.
+The zip has to contain a folder with the same name as the zip, case-sensitive, that contains what would normally be in
+the world's folder in `worlds/`. I.e. `worlds/ror2.apworld` containing `ror2/__init__.py`.
+
+
+## Metadata
+
+No metadata is specified yet.
+
+
+## Extra Data
+
+The zip can contain arbitrary files in addition what was specified above.