Adding external PR guidance to CONTRIBUTING.md (rebased onto dev_5_0)



This is the same as gh-1364 but rebased onto dev_5_0.

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See https://trello.com/c/NK9edzdd/205-add-handling-external-prs-to-contributing-doc-esp-for-bf

I will also add text to https://www.openmicroscopy.org/site/support/contributing/code-contributions.html in a PR against the docs repo.