README.md: Link to CONTRIBUTING.md

On Fri, Jan 10, 2014 at 09:55:53AM -0800, Aron Ahmadia wrote:
> My only suggestion is that we keep a link from the README back to
> CONTRIBUTING, so somebody skimming/searching the front page knows
> where to go next.

On Fri, Jan 10, 2014 at 10:31:04AM -0800, Justin Kitzes wrote:
> In response to @ahmadia comment, I would suggest removing the final
> sentence "Please submit corrections or additions as pull requests,
> bug reports, or by email to admin@software-carpentry.org." from the
> opening paragraph of the README and replacing it with "To contribute
> corrections or additions to this repository, see the contributing
> guidelines." with the appropriate link.