Readme needs an update

[eike-fokken]

Depending on the outcome of #278, probably the section
"nlohmann-json integration"
in the readme needs an update. It claims, that simply including nlohmann_json as a submodule is enough for the validator (also included as a submodule) to pick it up.

[pboettch]

This is still true, I'd love to have @LecrisUT to work on it as he masters fetch-content in cmake in a way I can learn things.