title: Fill a new pandas column with row numbers
body: <p>I have the following DataFrame <code>data</code> with random index values:</p>
  <pre><code>      A   B
  100   0   7
  203   5   4
  5992  0  10
  2003  9   8
  20   10   5
  12    6   2
  </code></pre>
  <p>I would like to add a new column 'C' with row numbers. For example:</p>
  <pre><code>      A   B   C
  100   0   7   0
  203   5   4   1
  5992  0  10   2
  2003  9   8   3
  20   10   5   4
  12    6   2   5
  </code></pre>