code: public static PropertyFile readOnly(final File filename) throws IOException
  	{
  		final PropertyFile props = new PropertyFile(filename);
  
  		props.readOnly = true;
  
  		return props;
  	}
docs: Creates a read-only version of a property file<br />
  Fails if the file does not exist.
  
  @param filename
  the filename to load
  
  @return
  
  @throws IOException
  if the file cannot be loaded