Adding external PR guidance to CONTRIBUTING.md
merge dev_5_0 -Dorg -Ssuccess-only: PR 1367 (Adding external PR guidance to CONTRIBUTING.md (rebased onto dev_5_0))
merge dev_5_0 -Dorg -Ssuccess-only

Repository: openmicroscopy/bioformats
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Merged PRs:
  # PR 1367 hflynn 'Adding external PR guidance to CONTRIBUTING.md (rebased onto dev_5_0)'

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