title: combine DataFrame MultiIndex to string column
body: <p>I have following DataFrame:</p>
  
  <pre><code>df = pd.DataFrame([[1,2,3], [11,22,33]], columns = ['A', 'B', 'C'])
  df.set_index(['A', 'B'], inplace=True)
  
          C
  A  B     
  1  2    3
  11 22  33
  </code></pre>
  
  <p>How I make additional 'text' column that will be string combination of the MultiIndex.</p>
  
  <p>Without removing my index!</p>
  
  <p>For example:</p>
  
  <pre><code>        C    D
  A  B            
  1  2    3    1_2
  11 22  33  11_22
  </code></pre>