feat(CONTRIBUTING): add design proposal guidelines

ripped straight from https://github.com/docker/docker/blob/master/CONTRIBUTING.md#design-and-cleanup-proposals
chore(Makefile): update development registry image to v0.9.0 tag
Merge pull request #2592 from mboersma/bump-dev-registry-0.9.0

chore(Makefile): update development registry image to v0.9.0 tag
Merge pull request #2576 from bacongobbler/design-proposal

feat(CONTRIBUTING): add design proposal guidelines
Merge remote-tracking branch 'upstream/master'

* upstream/master:
  chore(Makefile): update development registry image to v0.9.0 tag
  feat(CONTRIBUTING): add design proposal guidelines