title: how to rebalance mongodb after adding a shard
body: <p>How do I rebalance mongodb data after adding a new shard</p>
  
  <p>I do not think it does it automatically?</p>
  
  <p>I could not find anything, it's there support for this?</p>
  
  <p>Also the key for my shards is unique</p>
  
  <p>Thanks,</p>