Clarify in the native section that reason needs to be added a dependency

[Risto-Stevcev]

The Native section only talks about esy, but for those who aren't using esy it doesn't clarify that reason needs to be added as a dependency to the package under the depends stanza in dune-project.

I think this should be clarified in the docs for those that are just using the plain ocaml worklow and not esy.