README resolver addresses_for example improvements

[tadman]

The example given in the README is missing which require headers are necessary. Based on the errors you get when trying to run it bare the following would make sense:

require 'async'
require 'async/io/endpoint'
require 'async/dns/resolver'

Yet this results in:NoMethodError: undefined method each' for Async::IO::Endpoint:Class`

What's missing is:

require 'async'
require 'async/io'
require 'async/dns/resolver'

Ideally Async::DNS would just load whatever it needs as dependencies, at least if that doesn't cause any trouble. It'd be nice if require 'async/dns' or require 'async/dns/resolver' was sufficient.