title: Add a ranking ordered column to a Pandas Dataframe
body: <p>I know this question may seem trivial, but I can't find the solution anywhere. I have a really large pandas dataframe <code>df</code> that looks something like this:</p>
  
  <pre><code>                                            conference     IF2013  AR2013
  0                                            HOTMOBILE  16.333333   31.50
  1                                                 FOGA  13.772727   60.00
  2                                              IEA/AIE  10.433735   28.20
  3    IEEE Real-Time and Embedded Technology and App...  10.250000   29.00
  4                  Symposium on Computational Geometry   9.880342   35.00
  5                                                 WISA   9.693878   43.60
  6                                                 ICMT   8.750000   22.00
  7                                              Haskell   8.703704   39.00
  </code></pre>
  
  <p>I would like to add an extra column at the end that orders it 1,2,3,4, etc. So it looks like this:</p>
  
  <pre><code>                                               conference     IF2013    AR2013  Ranking 
      0                                            HOTMOBILE  16.333333   31.50   1  
      1                                                 FOGA  13.772727   60.00   2
      2                                              IEA/AIE  10.433735   28.20   3
      3    IEEE Real-Time and Embedded Technology and App...  10.250000   29.00   4  
  </code></pre>
  
  <p>I can't seem to figure out how to add a filled extra column that just put a Series of consecutive numbers.</p>