title: Inserting row in pandas dataframe based on date using datetime index
body: <pre><code>               col_A    vi_B    data_source index_as_date
  2017-01-21  0.000000  0.199354         sat       2017-01-21
  2017-01-22  0.000000  0.204250         NaN           NaT
  2017-01-23  0.000000  0.208077         NaN           NaT
  2017-01-27  0.000000  0.215081         NaN           NaT
  2017-01-28  0.000000  0.215300         NaN           NaT
  </code></pre>
  
  <p>In the pandas dataframe above, I want to insert a row for 24th January 2017 with value of <code>0.01, 0.4, sat, NaT</code>, how do I do that? I could use iloc and manually insert but I would prefer an automated solution which takes the datetime index into account</p>