title: multiple dataframe columns to list
body: <p>input is df:</p>
  
  <pre><code>   A  B
  0  a  d  
  1  b  e
  2  c  f
  </code></pre>
  
  <p>the output is a text file each row of df in a new line:</p>
  
  <p>df.txt</p>
  
  <pre><code>a, d
  b, e
  c, f
  </code></pre>
  
  <p>I can make it in an ugly way, what's the most efficient and pretty way of doing this?</p>