Add MAINTAINERS file.

Add MAINTAINERS file.

Previously, the list of committers was not written down publicly.  There
was no reason for this other than it not being trivial to expose the
commiter group membership via github.  This MAINTAINERS file lists the
members of the OVS committers group.

There's some outdated email addresses in here, but I just copied what
was currently in AUTHORS.  This can be fixed when AUTHORS gets fixed,
too.

Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>