Add FAQ for service worker docs

[gauntface]

Raising in response to: https://twitter.com/steveworkman/status/882957222120235008

Would be good to have an FAQ / Common pitfalls docs for service workers / offline support.

I found for the web push docs that an FAQ was a great place to cleanly phrase questions and answers or questions and link to specific content.

[vsr]

Spent quite sometime before figuring this out. I think this should go under the "Rough edges and gotchas" section at https://developers.google.com/web/fundamentals/getting-started/primers/service-workers

The solution for this seems to be finalized in the specs (w3c/ServiceWorker#893, w3c/ServiceWorker#1104). Firefox no longer caches service worker by default (https://bugzilla.mozilla.org/show_bug.cgi?id=1290944#c65).

While Chrome is implementing the solution (https://bugs.chromium.org/p/chromium/issues/detail?id=675540), could the article recommend disabling caching of Service Worker file?