Adding api overview

[NiklasVousten]

This adds a small info about the api in the README.md
Additionally it adds the openapi specification of the API.

See #32

[msrd0]

I don't think it is a good idea to add the openapi spec to the root of this repo. It is an auto-generated file, and those usually don't get placed in git sources, and especially not without CI checks to ensure they remain up to date.

[NiklasVousten]

But I think it would be good to have the API definition anywhere else then code or an own instance with docs enabled.
This was a simple attempt to add this.

Do you have any suggestions?

[msrd0]

What I usually do is ask CI to put stuff like auto generated documentation (usually the html from rustdoc) into the gh-pages branch. We could do something similar here, have CI generate the json and maybe even run redoc over it to get some html and publish to pages.