[gentoo-proxy-maint] New copyright policy and proxy-maint

[Michał Górny]

Hiya, everyone.

Since the new copyright policy [1] has been approved, I'd like to start
enforcing it for new proxy-maint contributions ASAP.  While it's not
enforced on ::gentoo yet, we tend to have some noticeable delay
in merging stuff, so I'd like to avoid suddenly realizing in N days that
we can't merge contributions submitted tomorrow because they haven't
followed the new policy yet.

At the same time, I think we should merge prior contributions as-is if
we manage to do it before the policy is actually enforced for
developers.  Nevertheless, if proxied maintainers feel like it, updating
old PRs to follow the policy is also a nice idea.

I've updated our guide to indicate the most important points
of the policy (others, please review and confirm I got it right!) [2]. 
I'll be updating Larry to verify sign-offs soonish.

Developers, please remember to append your sign-off to the submitter's
sign-off when you merge stuff!

[1]:https://www.gentoo.org/glep/glep-0076.html
[2]:https://wiki.gentoo.org/wiki/Project:Proxy_Maintainers/User_Guide#Copyright

-- 
Best regards,
Michał Górny