title: How can I add leading whitespace in front of the NSString?
body: <p>I need the String always have length 5...
  for example, if </p>
  
  <p>I pass <code>hello"</code>, I get <code>"hello"</code><br>
  I pass <code>"xml"</code>, I get <code>"  xml"</code> (two spaces before "xml");<br>
  I pass <code>"i"</code>, I get <code>"    i"</code> (four spaces before "i").</p>
  
  <p>What is the most efficient way to do it? Thank you.</p>