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 
I’ve updated it with the PR url, but searching for the string `Proposal:` is pointless with github search. We need to be more diligent about tagging them.


> On Nov 12, 2014, at 3:05 PM, Michael Crosby <notifications@github.com> wrote:
> 
> In CONTRIBUTING.md:
> 
> > @@ -23,10 +23,11 @@ reports to [security@docker.com](mailto:security@docker.com) instead.
> >  When considering a design proposal, we are looking for:
> >  
> >  * A description of the problem this design proposal solves
> > -* An issue -- not a pull request -- that describes what you will take action on
> > +* A pull request -- not an issue -- that modifies the documentation describing
> > +  the feature you are proposing, adding new ones if necessary.
> >    * 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)
> https://github.com/docker/docker/issues?
> 
> —
> Reply to this email directly or view it on GitHub <https://github.com/docker/docker/pull/9116/files#r20259735>.
>