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