Update CONTRIBUTING to reflect new PR-docs process.

This updates the contributing process/docs to adjust it to reflect the new "docs first" PR process. No new proposals are being made here.

ping @shykes 
We’re aware of that. It’s a process problem. The link should currently work and will continue to work.

> On Nov 12, 2014, at 3:52 PM, Sebastiaan van Stijn <notifications@github.com> wrote:
> 
> In CONTRIBUTING.md:
> 
> >    * Please prefix your issue with `Proposal:` in the title
> > -* Please review [the existing Proposals](https://github.com/docker/docker/issues?direction=asc&labels=Proposal&page=1&sort=created&state=open)
> > -  before reporting a new issue. You can always pair with someone if you both
> > +* Please review [the existing Proposals](https://github.com/docker/docker/pulls?q=is%3Aopen+is%3Apr+label%3AProposal)
> This link will only show PRs that are have a label proposal. Many proposals exist that are not (yet) labeled, but do start with "Proposal:" (https://github.com/docker/docker/pulls?q=is%3Apr+is%3Aopen+proposal <https://github.com/docker/docker/pulls?q=is%3Apr+is%3Aopen+proposal>)
> 
> Not sure if GH allows this, but make the link look for both PRs with "label:proposal" or "start with proposal"
> 
> Additionally, to prevent people creating proposals that have been rejected in the past, a short snippet "also look for closed proposals"?
> 
> —
> Reply to this email directly or view it on GitHub <https://github.com/docker/docker/pull/9116/files#r20262580>.
>