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>
Move 'hack' to the less confusing 'project'

We might want to break it up into smaller pieces (eg. tools in one
place, documents in another) but let's worry about that later.

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