title: Mongoose use only createdAt timestamp
body: <p>I have the following message schema in mongoose:</p>
  
  <pre><code>var messageSchema = mongoose.Schema({
    userID: { type: ObjectId, required: true, ref: 'User' },
    text:   { type: String, required: true }
  },
  {
    timestamps: true
  });
  </code></pre>
  
  <p>Is there anyway to ignore the updatedAt timestamp? Messages won't be updated so updatedAt will be wasted space</p>