title: R: frequency with group by ID
body: <p>I have a data frame like this: </p>
  
  <pre><code>ID Cont
  1   a
  1   a
  1   b
  2   a
  2   c
  2   d
  </code></pre>
  
  <p>I need to report the frequence of "Cont" by ID. The output should be</p>
  
  <pre><code>ID Cont Freq
  1   a    2
  1   b    1
  2   a    1
  2   c    1
  2   d    1
  </code></pre>