Document relationship with Polymer/pwa-starter-kit

[dandv]

Easy for our prospective users to be confused between https://polymer.github.io/pwa-starter-kit/what's-in-the-box/ and https://www.polymer-project.org/3.0/start/toolbox/set-up since they both provide service worker etc.

[keanulee]

Basically polymer-starter-kit is to Polymer library what pwa-starter-kit is to LitElement. pwa-starter-kit is what we're focused on moving forward.