title: How to add string to all values in a column of pandas DataFrame
body: <p>Say you have a <code>DataFrame</code> with columns;</p>
  
  <pre><code> col_1    col_2 
     1        a
     2        b
     3        c
     4        d
     5        e
  </code></pre>
  
  <p>how would you change the values of <code>col_2</code> so that, <em>new value</em> = <em>current value</em> + <code>'new'</code></p>