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

The main rationale is to make the 'Signed-off-by:'
requirement official. The file is copied from
the main OVS repo with some modifications.

Signed-off-by: Gurucharan Shetty <a1872e333d0e52644f6125da2276530f7ebe5e77@ovn.org>

shettyg/ovn-kubernetes,shettyg/ovn-kubernetes,shettyg/ovn-kubernetes