title: How to check whether a file or directory exists?
body: <p>I want to check the existence of file <code>./conf/app.ini</code> in my Go code, 
  but I can't find a good way to do that.  </p>
  
  <p>I know there is a method of File in Java: <code>public boolean exists()</code>, which returns true if the file or directory exists.</p>
  
  <p>But how can this be done in Go?</p>