title: Add column with values depending on another column to a dataframe
body: <p>I have got a dataframe to which I want to append a column with values depending on a column in my dataframe.
  My dataframe looks somewhat like this:</p>
  
  <pre><code>c1  c2  c3
  x   2   z
  y   5   f
  c   3   r
  a   11  z
  </code></pre>
  
  <p>Now I want to append another column <code>c4</code> based on the values of <code>c2</code>.
  For all values between 0 and 4 I want to append <code>"low"</code>, for values between 5 and 9 I want to append <code>"medium"</code> and for those bigger than 10 <code>"high"</code>.</p>
  
  <pre><code>c1  c2  c3  c4
  x   2   z   "low"
  y   5   f   "medium"
  c   3   r   "low"
  a   11  z   "high"
  </code></pre>
  
  <p>Probably the answer is quite simple, but I really can't think of something.</p>