README: improve browser setup advice

As per https://github.com/bitcoinjs/bitcoinjs-lib/issues/314, this updates the README to include more up-to-date and idiomatic build advice for users who want to use the library from the browser.

Suggestions welcome and appreciated.
@kyledrake I'm personally not a fan of this remaining since it provides a user with a `bitcoinjs-lib` that isn't entirely usable as they would be missing particular dependencies to use some parts of our API.
We do allow duck typing which means they could include the other dependencies as individual minified files if they so wish; but it does encourage a bad practice IMHO.