title: How to add percent sign in string format?
body: <p>I want to get string like <code>"99.99%"</code> from <code>Double</code>,and I used format to get this:</p>
  
  <pre><code>let rate = 99.99999
  let str = String(format: "%.2f%", rate)
  // output 99.99
  </code></pre>
  
  <p>And <code>\%</code> is not allowed. So how to add percent sign in string format, please help me!</p>