Update Secure Context section; add Implementer Concerns section.
Update Secure Context section; add Implementer Concerns section.

slightlyoff/ServiceWorker,justinleoye/ServiceWorker,justinleoye/ServiceWorker,martinthomson/ServiceWorker,chadoliver/ServiceWorker,mkruisselbrink/ServiceWorker,mkruisselbrink/ServiceWorker,zuzusik/ServiceWorker,justinleoye/ServiceWorker,chadoliver/ServiceWorker,martinthomson/ServiceWorker,zuzusik/ServiceWorker,zuzusik/ServiceWorker,martinthomson/ServiceWorker,slightlyoff/ServiceWorker,chadoliver/ServiceWorker