Create "sandbox" repository for new contributors

[thaJeztah]

We should have a repository that new contributors can use to create pull-requests on, to learn the contributing procedures. The "contributing guide" should use that repository in the examples (to prevent users from accidentally creating PRs against the "real" repositories).

Gordon/Leeroy can automatically respond to those PR's, and do fun things like "congratulate" them on a successful pull request, teach how to sign-off commits, rebase, etc.

For some background, see https://github.com/docker/docker/pull/17330#issuecomment-150939264

/cc @moxiegirl @SvenDowideit