CONTRIBUTING: Document Signed-off-by and the DCO

[wking]

This project seems to have been operating under the assumption that
the usual Signed-off-by ↔ DCO mapping applied. But it's nice to
make that explicit. This commit copies the current DCO section from
runtime-spec, but puts it in CONTRIBUTING.md for a more native GitHub
experience.

Looking through the history of the repo, the only non-merge commits
without sign-offs are by Mrunal:

• 5286bb0 (Update runtime validation section, 2016-01-19)
• b71f045 (Update README.md, 2015-11-09)
• 2c0e9e2 (Update help for generate, 2015-11-05)
• 850abe9 (Update README.md, 2015-09-15)
• bb90389 (Initial commit, 2015-09-15)

who clearly doesn't have a problem signing for the rest of his work
and is unlikely to have stolen those contributions from somewhere else
;).

[liangchenye]

:) LGTM
@mrunalp

[liangchenye]

@wking I have a question about the CONTRIBUTING thing.
If someone else wrote a patch and I want to submit it for him/her due to some reasons,
is it sufficient that I add his/her name to the commit?

[wking]

On Wed, Apr 27, 2016 at 11:44:41PM -0700, 梁辰晔 (Liang Chenye) wrote:
“If someone else wrote a patch and I want to submit it for him/her
due to some reasons, is it sufficient that I add his/her name to the
commit?”

The “; or” bits mean that you have to hit one of (a), (b), or (c)
(you always have to hit (d)). I think (b) allows you to submit
someone else's appropriately-licensed code without their Signed-off-by
(see also discussion around 1). But it certainly seems like better
practice to have them add their own Signed-off-by, followed by you
adding your Signed-off-by (more notes on this in 1 as well).

[liangchenye]

Thanks @wking

[mrunalp]

Thanks 😃 LGTM