Update CONTRIBUTING to reflect new PR-docs process.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Merge pull request #9116 from erikh/update_contributing

Update CONTRIBUTING to reflect new PR-docs process.
Improve "security reports" section in contribution docs.

Signed-off-by: Solomon Hykes <solomon@docker.com>
In contribution docs, emphasize that bug reports are appreciated!

Signed-off-by: Solomon Hykes <solomon@docker.com>
Move GOVERNANCE.md to hack/

Signed-off-by: Solomon Hykes <solomon@docker.com>