chore(Docs): Adds Firebase instructions to PWA example.
chore(Docs): Adds Firebase instructions to PWA example.

Adds Firebase instructions to PWA example

Closes #947

PiperOrigin-RevId: 187119175

angulardart/angular,chalin/angular2,kevmoo/angular,chalin/angular2,angulardart/angular,matanlurey/angular,angulardart/angular,kevmoo/angular,kevmoo/angular,chalin/angular2,matanlurey/angular,matanlurey/angular