Add note about using mdx with mli files

[mooreryan]

By default, Dune only processes markdown files with mdx. This note gives an example of how to use Dune and mdx in mli files.

It's just a small change to the readme, but if it was there, it would have saved me a few minutes skimming through the Dune docs to figure out what to do. Do you think it is a helpful addition to the readme?

[Leonidas-from-XIV]

Sounds good to me, thanks for your contribution!