title: How to determine RestartStrategy with Dynamic Supervision
body: <p>If start child spec statically： </p>
  
  <pre><code>init(...) -&gt;
      {ok, {{RestartStrategy, MaxR, MaxT},
            [ChildSpec, ...]}}.
  </code></pre>
  
  <p>But if I start child spec dynamically:</p>
  
  <pre><code>start_child(SupRef, ChildSpec) -&gt; startchild_ret()
  </code></pre>
  
  <p>How to determine RestartStrategy、 MaxR and MaxT ?</p>