title: Concatenate Pandas dataframes with different set of columns
body: <p>df1 has columns A, B, C, D, E</p>
  <p>df2 has columns A, B, D</p>
  <p>How to concatenate them in order to have a resulting dataframe that has rows of df1 and df2, values of A, B and D will be extended from df2 on df1, and columns C and E will be filled with NaN because df2 has no data for them?</p>