Light community contributors
More lightweight community contributor tier under Core Team is suggested.
Currently becoming a Core Team member has relatively high barrier, but candidates to Core should be more encouraged or pooled.
For such purpose, add community contributors to "member" with 3-5 MRs contributions.
The tier name should be considered.
cf. https://github.com/kubernetes/community/blob/master/community-membership.md
This would be better after gitlab-ce#49860 is done.