OPAM package

OPAM is a widely used package manager to distribute Coq developments. Do we want to put a package for Prosa on the Coq repository ? (http://coq.io/opam/)

This is mostly a matter of writing a short opam file and submitting a PR to this repo https://github.com/coq/opam-coq-archive I can take care of all that, if we agree to do it.