Mention pluginify in plugin authoring docs

[itowlson]

Fixes #1386. @michelleN I wasn't quite sure how to fit it into the explanation of the packaging flow and I think this might be too far into the "where no-one can find it" zone. Although in practice I think most plugin devs will find out about it by copying other plugins rather than by reading the docs so 🤷. What do you reckon?

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Pre-Merge Content Checklist

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