add log message for service worker doc

[viankakrisna]

Motivation: #2398

Right now a lot of people is confused because they suddenly have service worker in production. Rather than letting people shoot their feet, i think we need to add this message to dev environment so they will know beforehand that there will be caching etc for PWA on production builds and can make the informed decision before running npm run build

screenshot of how it look in the console

[vpavlenko]

@viankakrisna Thanks, that's a great step towards lowering everyone's confusion with Service Workers. Wouldn't it be more helpful to add an info about it and the possibility to disable it right to the stub content, after "To get started, ..."?

[viankakrisna]

@vpavlenko as there is current work to disable service-worker by default #3817, I think this PR is no longer necessary