add to key dataframe code example

Example: pandas append index ignore

>>> df = pd.DataFrame([[1, 2], [3, 4]], columns=list('AB'))
>>> df2 = pd.DataFrame([[5, 6], [7, 8]], columns=list('AB'))
>>> df.append(df2, ignore_index=True)
   A  B
0  1  2
1  3  4
2  5  6
3  7  8