Provide user-facing introduction about goal of peanut

[felixblaschke]

The current introduction ("Do you ever want to pub run build_runner build into another branch? This is your ticket.") sounds cool but is very technical. Your package (using default parameters) looks to me to have a clear goal (or default use case), like:

"Peanut builds your Flutter project as a webapp and puts the website sources as an extra branch, that is ready to be hosted on GitHub Pages."

This might gives your users a better idea what to expect from peanut.

I also think this use case is very common once Flutter web gets adopted: Host sources on GitHub and use it to host that webapp.