Merge pull request #1367 from hflynn/rebased/dev_5_0/external-prs

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

Repository: openmicroscopy/bioformats
Updating 440626c..77ddc6d
Previously merged:
  # PR 1367 hflynn 'Adding external PR guidance to CONTRIBUTING.md (rebased onto dev_5_0)'

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