title: Add a TOTAL row to a pandas dataframe
body: <p>I have a dataframe as follows:</p>
  
  <pre><code>                   POSITIVE  NEGATIVE  MASTER Net Positivity
  Speaker                                                     
  Mike                 36.0      29.0  2187.0          0.32%
  Rob                  36.0      35.0  2413.0          0.04%
  May                   2.0       4.0   369.0         -0.54%
  </code></pre>
  
  <p>I'd like to add a row at the end, which has the total positive (36+36+2), total negative (29+35+4), total MASTER (2187+2413+369). The index of the new row should be 'TOTAL'. The total net positivity is (total POSITIVE - total NEGATIVE) / total MASTER.</p>
  
  <pre><code>                   POSITIVE  NEGATIVE  MASTER Net Positivity
  Speaker                                                     
  Mike                 36.0      29.0  2187.0          0.32%
  Rob                  36.0      35.0  2413.0          0.04%
  May                   2.0       4.0   369.0         -0.54%
  TOTAL                74.0      68.0  4969.0          0.12%
  </code></pre>
  
  <p>How should I add this total row in pandas?</p>