title: Selecting columns equal to a substring in python
body: <h2>My dataframe looks like this</h2>
  
  <ol>
  <li>clients4.google.com</li>
  <li>m.facebook.com</li>
  <li>static.ak.fbcdn.net</li>
  <li>static.ak.fbcdn.net</li>
  <li>static.ak.fbcdn.net</li>
  </ol>
  
  <p>I want to select all the rows where the substring fb is available.
  In pandas dataframe</p>