maintainers/README: mention GitHub team creation (#446969)

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maintainers
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maintainers/README.md
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In any case, request reviews from the existing team members.
If the team lists no specific membership policy, feel free to merge changes to the team after giving the existing members a few days to respond.
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*Important:* If a team says it is a closed group, do not merge additions to the team without an approval by at least one existing member.
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> [!IMPORTANT]
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> If a team says it is a closed group, do not merge additions to the team without an approval by at least one existing member.
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A corresponding GitHub team can be created by any org member.
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When creating the team it should be created with the `nixpkgs-maintainers` team as parent.
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Once approved, the team will have the right privileges to be pinged and requested for review in Nixpkgs.
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> [!TIP]
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> The team name should be as short as possible; because it is nested under the maintainers group, no -maintainers suffix is needed.
# Maintainer scripts