Note the shared Flux v2 maintainers and team

Many of the GitOps Toolkit controllers will share maintainers with
this repo, acting as the central Flux v2 repo. For convenience of
tracking membership and applying permissions, we can share maintainers
by:

 - referring to the MAINTAINERS file here, from elsewhere;
 - making a GitHub team that tracks those shared maintainers.

This commit makes a note of this in the MAINTAINERS file, to inform
people of the arrangement, and remind them to keep the team up to
date.

Signed-off-by: Michael Bridgen <michael@weave.works>