title: Dictionary with list of strings as value
body: <p>I have a dictionary where my value is a List.
  When I add keys, if the key exists I want to add another string to the value (List)?
  If the key doesn't exist then I create a new entry with a new list with a value, if the key exists then I jsut add a value to the List value
  ex.</p>
  
  <pre><code>Dictionary&lt;string, List&lt;string&gt;&gt; myDic = new Dictionary&lt;string, List&lt;string&gt;&gt;();
  myDic.Add(newKey, add to existing list&lt;strings&gt; and not create new one)
  </code></pre>