title: Add a new column 'Score' to the DataFrame with values based on existing categorical column Medal
body: <p>Let's say: there is a data frame:</p>
  
  <pre><code>country       Edition     sports       Athletes           Medal  score
  Germany          1990    Aquatics  HAJOS, Alfred          gold     3
  Germany          1990    Aquatics  HIRSCHMANN, Otto       silver   2
  Germany          1990    Aquatics  DRIVAS, Dimitrios      silver   2
  US               2008    Athletics MALOKINIS, Ioannis     gold     3
  US               2008    Athletics HAJOS, Alfred          silver   2
  US               2009    Athletics CHASAPIS, Spiridon     gold     3
  France           2010    Athletics CHOROPHAS, Efstathios  gold     3
  France           2010    Athletics CHOROPHAS, Efstathios  gold     3
  France           2010    golf      HAJOS, Alfred          Bronze   1
  France           2011    golf      ANDREOU, Joannis       silver   2
  Spain            2011    golf      BURKE, Thomas          gold     3
  </code></pre>
  
  <p>bronze =1 ,silver =2 ,gold =3. </p>
  
  <p>can anyone tell me how to assign these values to new column <code>'score'</code>? after adding new column <code>df['score']</code>? </p>