title: How to add own index to pip running in virtualenv?
body: <ul>
  <li>I have a virtualenv with pip inside.</li>
  <li>I have my own package index.</li>
  <li>I want to add this package to pip to avoid invoking <code>pip install some_package -i my_index</code> all the time</li>
  <li>I want to add this index only to this specific pip instance running in virtualenv.</li>
  </ul>
  
  <p>In that case, which file should I modify and how?</p>