Add a jbuilder transition package

[ghost]

This should help with #792. Now when running opam pin add jbuilder --dev, the installation fails if dune is not installed with the following message:

=-=- jbuilder.transition troubleshooting =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
=> Jbuilder has been renamed and the jbuilder package is now a transition package.
=> Use the dune package instead.
=> 
=> If you want to try the development version of Dune before the 1.0.0 is released, run the following command:
=> 
=> $ opam pin add dune https://github.com/ocaml/dune.git

[avsm]

Maybe wrap this line to 76 cols

[ghost]

make test finds a cycle between the jbuilder and dune packages. I wonder if it's not because of the README.md and other files we automatically install

[bobbypriam]

CMIIW, shouldn't this be transitions? (or the one in dune.opam should be transition.)

[ghost]

Indeed, it's the one in dune.opam that's wrong. Thanks for pointing that out