Update co-maintainers list

Add Milan, Phaedrus and Philip as co-maintainers, and expand the README slightly to document how the co-maintainership model is expected to work. Basically: we ack each others’ patches, and any major decisions need 1 or 2 other people’s input. For even bigger decisions we may also want to pull in @robert.ancell from Canonical. Design decisions can be made by @aday.

@hughsie’s plan is to reduce his involvement over the next 5 months.

In terms of releases, @hughsie will continue to do the current stable branch releases, and the other three of us need to decide between ourselves about which of us will do the unstable releases in future. We could coordinate on IRC, chat.gnome.org or discourse.gnome.org — what are your preferences, @mcrha, @mwleeds?

I looked at using gitlab’s milestones or releases for tracking who’s going to do a particular release, but there’s no specific way of doing that. Nevertheless, I think using the milestones for rough release planning (tracking what issues/MRs need to land before a given release) would make sense.