add complete app example of lwt_reporter with library

[rbjorklin]

This provides a complete example of an app with library that uses logs. Fixes #45

I know you don't personally use dune but I hope it is okay to include dune files in the example as it is widely used within the community.

[dbuenzli]

Thanks for this. I will have a look and integrate it differently when I get some time.

I rather have that in the docs like is done in brr. So that people can read that from odig or ocaml.org rather than having to hunt for the repo.