Update CONTRIBUTING.md
Update CONTRIBUTING.md

amend for Carpentries

Former-commit-id: 9807ff2240a5b2db6321f17227b599fa81356e77 [formerly 0cc4ffc3ae8a507273605da223894063a2f1799f]
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LibraryCarpentry/librarycarpentry.github.io,LibraryCarpentry/librarycarpentry.github.io,LibraryCarpentry/librarycarpentry.github.io