title: Concat two DataFrames on missing indices
body: <p>I have two DataFrames and want to use the second one only on the rows whose index is not already contained in the first one.</p>
  
  <p>What is the most efficient way to do this?</p>
  
  <p><strong>Example:</strong></p>
  
  <pre><code>df_1
  idx     val
  0      0.32
  1      0.54
  4      0.26
  5      0.76
  7      0.23
  
  df_2
  idx     val
  1     10.24
  2     10.90
  3     10.66
  4     10.25
  6     10.13
  7     10.52
  
  df_final
  idx     val
  0      0.32
  1      0.54
  2     10.90
  3     10.66
  4      0.26
  5      0.76
  6     10.13
  7      0.23
  </code></pre>
  
  <p>Recap: I need to add the rows in <code>df_2</code> for which the index is not already in <code>df_1</code>.</p>
  
  <hr>
  
  <p><strong>EDIT</strong></p>
  
  <p>Removed some indices in <code>df_2</code> to illustrate the fact that all indices from <code>df_1</code> are not covered in <code>df_2</code>.</p>