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 ?

_When reporting bugs, please include relevant JavaScript Console logs and links to public URLs at which the issue could be reproduced._