Migration guide for v6 -> v7 ?

Welcome! Please use this template for reporting bugs or requesting features. For questions about using Workbox, the best place to ask is Stack Overflow, tagged with [workbox]: https://stackoverflow.com/questions/ask?tags=workbox

Library Affected: workbox-sw, workbox-build, etc.

• v5 workbox-* libraries (we depend on 13 of them)

Browser & Platform: E.g. Google Chrome v51.0.1 for Android, or "all browsers".

• all browsers

Issue or Feature Request Description: Your request here.

• Is there a migration guide for v6->v7 ? Or are there breaking changes listed somewhere ?

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