Add a CONTRIBUTING.md file
Add a CONTRIBUTING.md file

Signed-off-by: Doug Davis <ff3bde741799c0eff815b83a54eda3fc494df54f@us.ibm.com>

projectatomic/skopeo,mtrmac/skopeo,projectatomic/skopeo,mtrmac/skopeo,mikebrow/skopeo,mikebrow/skopeo,jwhonce/skopeo,jwhonce/skopeo,mtrmac/skopeo