Add opam pin support

[kit-ty-kate]

As an opam user i would like to do opam pin add https://github.com/rossberg/camp.git to install your project, however currently this doesn't work because your project doesn't contain any standard opam file.

• The first commit adds an opam file based on the already present description that you've added in the dune-project
• The second commit is optional, but i personally don't think duplicating the packaging information in the dune-project makes any sense past the first generation. But this can be a matter of taste and if you disagree feel free to remove that commit, i really don't mind.