The previous details were showing their age, e.g. mentions about browsers
not supporting ES2015. The story with ES modules is more complex as it's also
about loaders but to keep the README simple, let's just make it more up to date
with typical usage.
Closes gh-5108
Use a dist README fixture kept in the jQuery repository instead of modifying
an existing one. This makes the jQuery repository the single source of truth
when it comes to jQuery releases and it makes it easier to make changes to
README without worrying how it will affect older jQuery lines.
The commit also ES6ifies build/release.js & build/release/dist.js
Closes gh-4614
