title: Insert rows on specific line in a file
body: <p>I have opened my existing file in r+ mode.</p>
  
  <p>open("#{RAILS_ROOT}/locale/app.pot", 'r+') do |f|</p>
  
  <p>end</p>
  
  <p>I want to insert some other rows at specific line no..
  Like i want insert "Hii " on line number 10.
  "hello " on line number 2.
  "world " on line number 20.</p>
  
  <p>How may i handle it in ruby ??</p>