title: Add column for percentage of total to Pandas dataframe
body: <p>I have a dataframe that I am doing a <code>groupby()</code> on to get the counts on a column's values. I am trying to add an additional column for "Percentage of Total".  I'm not sure how to accomplish that.  </p>
  
  <p>I've looked at a few groupby options, but can't seem to find anything that fits. </p>
  
  <p>My dataframe looks like this:</p>
  
  <pre><code>              DAYSLATE
  DAYSLATE          
  -7 days          1
  -5 days          2
  -3 days          8
  -2 days          9
  -1 days         45
  0 days         589
  1 days          33
  2 days           8
  3 days          16
  4 days          14
  5 days          16
  6 days           2
  7 days           6
  8 days           2
  9 days           2
  10 days          1
  </code></pre>