title: Insert a row to pandas dataframe
body: <p>I have a dataframe:</p>
  
  <pre><code>s1 = pd.Series([5, 6, 7])
  s2 = pd.Series([7, 8, 9])
  
  df = pd.DataFrame([list(s1), list(s2)],  columns =  ["A", "B", "C"])
  
     A  B  C
  0  5  6  7
  1  7  8  9
  
  [2 rows x 3 columns]
  </code></pre>
  
  <p>and I need to add a first row [2, 3, 4] to get:</p>
  
  <pre><code>   A  B  C
  0  2  3  4
  1  5  6  7
  2  7  8  9
  </code></pre>
  
  <p>I've tried <code>append()</code> and <code>concat()</code> functions but can't find the right way how to do that.</p>
  
  <p>How to add/insert series to dataframe?</p>