Update README.md

Author: rain0r

README.md

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 This project aims to provide an OpenAPI 3 specification for the [ListenBrainz API](https://listenbrainz.readthedocs.io/en/latest/users/api/index.html#reference).
 
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 ## Structure
 
 The specification is split up in multiple folders and files. Here is an overview of the folders: