Document how to use preludes in the README

[samoht]

It would be nice to summarise what the tests are doing in the REAMDE, e.g.

• you can either use a file and pass --prelude x.ml to mdx like in https://github.com/realworldocaml/mdx/tree/main/test/bin/mdx-test/expect/prelude-file
• or pass an OCaml expression directly to mdx e.g. --prelude-str "let x = 3" like in https://github.com/realworldocaml/mdx/blob/main/test/bin/mdx-test/expect/prelude

And you can choose to load the prelude on one one environnement with --prelude "env:file.ml"

[yminsky]

Would also be good to document how to invoke said features from Dune.