title: add new vocabulary to existing Doc2vec model
body: <p>I Already have a Doc2Vec model. I have trained it with my train data.</p>
  
  <p>Now after a while I want to use Doc2Vec for my test data. I want to add my test data vocabulary to my existing model's vocabulary. How can I do this?
  I mean how can I update my vocabulary?</p>
  
  <p>Here is my model:</p>
  
  <pre><code>    model = model.load('my_model.Doc2vec')
  </code></pre>