Added PWA manifest plugin and default configuration + Updated README

[rubenaguadoc]

Maybe this is useful

[tr1s]

I was playing around with it, but after some reading, I think I'm going to opt out of this PR. The webpack-pwa-manifest is kind of abandoned, and I don't want to add extra complexity and confusion that might not get resolved in the future.

The idea here was that I'll prepare the boilerplate for people that want to take the full plunge into it themselves.

I'd consider adding in a barebones manifest.json for the sake of it, or if there's a more robust way to inject it that would be nice. I'll look into it more and feel to do the same and let me know your thoughts.