title: Make new column in Panda dataframe by adding values from other columns
body: <p>I have a dataframe with values like </p>
  
  <pre><code>A B
  1 4
  2 6
  3 9
  </code></pre>
  
  <p>I need to add a new column by adding values from column A and B, like</p>
  
  <pre><code>A B C
  1 4 5
  2 6 8
  3 9 12
  </code></pre>
  
  <p>I believe this can be done using lambda function, but I can't figure out how to do it.</p>