title: Pandas: Appending a row to a dataframe and specify its index label
body: <p>Is there any way to specify the index that I want for a new row, when appending the row to a dataframe?</p>
  
  <p>The original documentation provides <a href="http://pandas.pydata.org/pandas-docs/stable/merging.html">the following example</a>:</p>
  
  <pre><code>In [1301]: df = DataFrame(np.random.randn(8, 4), columns=['A','B','C','D'])
  
  In [1302]: df
  Out[1302]: 
            A         B         C         D
  0 -1.137707 -0.891060 -0.693921  1.613616
  1  0.464000  0.227371 -0.496922  0.306389
  2 -2.290613 -1.134623 -1.561819 -0.260838
  3  0.281957  1.523962 -0.902937  0.068159
  4 -0.057873 -0.368204 -1.144073  0.861209
  5  0.800193  0.782098 -1.069094 -1.099248
  6  0.255269  0.009750  0.661084  0.379319
  7 -0.008434  1.952541 -1.056652  0.533946
  
  In [1303]: s = df.xs(3)
  
  In [1304]: df.append(s, ignore_index=True)
  Out[1304]: 
            A         B         C         D
  0 -1.137707 -0.891060 -0.693921  1.613616
  1  0.464000  0.227371 -0.496922  0.306389
  2 -2.290613 -1.134623 -1.561819 -0.260838
  3  0.281957  1.523962 -0.902937  0.068159
  4 -0.057873 -0.368204 -1.144073  0.861209
  5  0.800193  0.782098 -1.069094 -1.099248
  6  0.255269  0.009750  0.661084  0.379319
  7 -0.008434  1.952541 -1.056652  0.533946
  8  0.281957  1.523962 -0.902937  0.068159
  </code></pre>
  
  <p>where the new row gets the index label automatically. Is there any way to control the new label?</p>