title: counting column values in pandas
body: <pre><code>dictionary = {'Year': [1985, 1985, 1986, 1986, 1987, 1987, 1987]}
  pdf = pd.DataFrame(dictionary)
  
      Year
  0   1985
  1   1985
  2   1986
  3   1986
  4   1987
  5   1987
  6   1987
  </code></pre>
  
  <p>I have a dataframe called <code>pdf</code> I need to form a <code>new data frame</code> in the following format:</p>
  
  <pre><code>Year   count
  1985     2
  1986     2 
  1987     3
  </code></pre>
  
  <p>How can do this efficiently in pandas? </p>