title: Add a ReadableDuration to a ReadableInstant
body: <p>Given a <code>ReadableInstant</code> and a <code>ReadableDuration</code>, I want to create another <code>ReadableInstant</code> that represents the point in time that lies the given duration after the given point in time. The simplest way to do this that I found is this but I assume there is some way to do this directly:</p>
  
  <pre><code>ReadableInstant after(ReadableInstant instant, ReadableDuration duration) {
      return new Instant(instant.getMillis() + duration.getMillis());
  }
  </code></pre>
  
  <p><code>Instant</code> defines a method <code>plus()</code> which takes a <code>ReadableDuration</code> but there are other implementations of <code>ReadableInstant</code> so I can't use that method.</p>