You seem to have done things right. The problem is that you have not (yet) gotten push access to METADATA.jl, and currently nobody has written the code that lets Pkg.publish() automatically fork it on github.com and create a pull request. This means that you manually have to fork METADATA.jl and push your changes there, before opening a pull request.
(I hope I didn't add to the confusion, please ask if you still have problems)
